mz_adapter/

session.rs

// Copyright Materialize, Inc. and contributors. All rights reserved.
//
// Use of this software is governed by the Business Source License
// included in the LICENSE file.
//
// As of the Change Date specified in that file, in accordance with
// the Business Source License, use of this software will be governed
// by the Apache License, Version 2.0.

//! Per-connection configuration parameters and state.

#![warn(missing_docs)]

use std::collections::btree_map::Entry;
use std::collections::{BTreeMap, BTreeSet};
use std::fmt::Debug;
use std::future::Future;
use std::mem;
use std::net::IpAddr;
use std::sync::Arc;

use chrono::{DateTime, Utc};
use derivative::Derivative;
use itertools::Itertools;
use mz_adapter_types::connection::ConnectionId;
use mz_build_info::{BuildInfo, DUMMY_BUILD_INFO};
use mz_controller_types::ClusterId;
use mz_ore::metrics::{MetricsFutureExt, MetricsRegistry};
use mz_ore::now::{EpochMillis, NowFn};
use mz_pgwire_common::Format;
use mz_repr::role_id::RoleId;
use mz_repr::user::{ExternalUserMetadata, InternalUserMetadata};
use mz_repr::{CatalogItemId, Datum, Row, RowIterator, ScalarType, TimestampManipulation};
use mz_sql::ast::{AstInfo, Raw, Statement, TransactionAccessMode};
use mz_sql::plan::{Params, PlanContext, QueryWhen, StatementDesc};
use mz_sql::session::metadata::SessionMetadata;
use mz_sql::session::user::{
    INTERNAL_USER_NAME_TO_DEFAULT_CLUSTER, RoleMetadata, SYSTEM_USER, User,
};
use mz_sql::session::vars::IsolationLevel;
pub use mz_sql::session::vars::{
    DEFAULT_DATABASE_NAME, EndTransactionAction, SERVER_MAJOR_VERSION, SERVER_MINOR_VERSION,
    SERVER_PATCH_VERSION, SessionVars, Var,
};
use mz_sql_parser::ast::TransactionIsolationLevel;
use mz_storage_client::client::TableData;
use mz_storage_types::sources::Timeline;
use qcell::{QCell, QCellOwner};
use rand::Rng;
use tokio::sync::mpsc::{self, UnboundedReceiver, UnboundedSender};
use tokio::sync::watch;
use uuid::Uuid;

use crate::AdapterNotice;
use crate::catalog::CatalogState;
use crate::client::RecordFirstRowStream;
use crate::coord::ExplainContext;
use crate::coord::appends::BuiltinTableAppendNotify;
use crate::coord::in_memory_oracle::InMemoryTimestampOracle;
use crate::coord::peek::PeekResponseUnary;
use crate::coord::statement_logging::PreparedStatementLoggingInfo;
use crate::coord::timestamp_selection::{TimestampContext, TimestampDetermination};
use crate::error::AdapterError;
use crate::metrics::{Metrics, SessionMetrics};

const DUMMY_CONNECTION_ID: ConnectionId = ConnectionId::Static(0);

/// A session holds per-connection state.
#[derive(Derivative)]
#[derivative(Debug)]
pub struct Session<T = mz_repr::Timestamp>
where
    T: Debug + Clone + Send + Sync,
{
    conn_id: ConnectionId,
    /// A globally unique identifier for the session. Not to be confused
    /// with `conn_id`, which may be reused.
    uuid: Uuid,
    prepared_statements: BTreeMap<String, PreparedStatement>,
    portals: BTreeMap<String, Portal>,
    transaction: TransactionStatus<T>,
    pcx: Option<PlanContext>,
    metrics: SessionMetrics,
    #[derivative(Debug = "ignore")]
    builtin_updates: Option<BuiltinTableAppendNotify>,

    /// The role metadata of the current session.
    ///
    /// Invariant: role_metadata must be `Some` after the user has
    /// successfully connected to and authenticated with Materialize.
    ///
    /// Prefer using this value over [`Self.user.name`].
    //
    // It would be better for this not to be an Option, but the
    // `Session` is initialized before the user has connected to
    // Materialize and is able to look up the `RoleMetadata`. The `Session`
    // is also used to return an error when no role exists and
    // therefore there is no valid `RoleMetadata`.
    role_metadata: Option<RoleMetadata>,
    client_ip: Option<IpAddr>,
    vars: SessionVars,
    notices_tx: mpsc::UnboundedSender<AdapterNotice>,
    notices_rx: mpsc::UnboundedReceiver<AdapterNotice>,
    next_transaction_id: TransactionId,
    secret_key: u32,
    external_metadata_rx: Option<watch::Receiver<ExternalUserMetadata>>,
    // Token allowing us to access `Arc<QCell<StatementLogging>>`
    // metadata. We want these to be reference-counted, because the same
    // statement might be referenced from multiple portals simultaneously.
    //
    // However, they can't be `Rc<RefCell<StatementLogging>>`, because
    // the `Session` is sent around to different threads.
    //
    // On the other hand, they don't need to be
    // `Arc<Mutex<StatementLogging>>`, because they will always be
    // accessed from the same thread that the `Session` is currently
    // on. We express this by gating access with this token.
    #[derivative(Debug = "ignore")]
    qcell_owner: QCellOwner,
    session_oracles: BTreeMap<Timeline, InMemoryTimestampOracle<T, NowFn<T>>>,
}

impl<T> SessionMetadata for Session<T>
where
    T: Debug + Clone + Send + Sync,
    T: TimestampManipulation,
{
    fn conn_id(&self) -> &ConnectionId {
        &self.conn_id
    }

    fn client_ip(&self) -> Option<&IpAddr> {
        self.client_ip.as_ref()
    }

    fn pcx(&self) -> &PlanContext {
        &self
            .transaction()
            .inner()
            .expect("no active transaction")
            .pcx
    }

    fn role_metadata(&self) -> &RoleMetadata {
        self.role_metadata
            .as_ref()
            .expect("role_metadata invariant violated")
    }

    fn vars(&self) -> &SessionVars {
        &self.vars
    }
}

/// Data structure suitable for passing to other threads that need access to some common Session
/// properties.
#[derive(Debug)]
pub struct SessionMeta {
    conn_id: ConnectionId,
    client_ip: Option<IpAddr>,
    pcx: PlanContext,
    role_metadata: RoleMetadata,
    vars: SessionVars,
}

impl SessionMetadata for SessionMeta {
    fn vars(&self) -> &SessionVars {
        &self.vars
    }

    fn conn_id(&self) -> &ConnectionId {
        &self.conn_id
    }

    fn client_ip(&self) -> Option<&IpAddr> {
        self.client_ip.as_ref()
    }

    fn pcx(&self) -> &PlanContext {
        &self.pcx
    }

    fn role_metadata(&self) -> &RoleMetadata {
        &self.role_metadata
    }
}

/// Configures a new [`Session`].
#[derive(Debug, Clone)]
pub struct SessionConfig {
    /// The connection ID for the session.
    ///
    /// May be reused after the session terminates.
    pub conn_id: ConnectionId,
    /// A universally unique identifier for the session, across all processes,
    /// region, and all time.
    ///
    /// Must not be reused, even after the session terminates.
    pub uuid: Uuid,
    /// The peer address of the client
    pub client_ip: Option<IpAddr>,
    /// The name of the user associated with the session.
    pub user: String,
    /// An optional receiver that the session will periodically check for
    /// updates to a user's external metadata.
    pub external_metadata_rx: Option<watch::Receiver<ExternalUserMetadata>>,
    /// The metadata of the user associated with the session.
    pub internal_user_metadata: Option<InternalUserMetadata>,
    /// Helm chart version
    pub helm_chart_version: Option<String>,
}

impl<T: TimestampManipulation> Session<T> {
    /// Creates a new session for the specified connection ID.
    pub(crate) fn new(
        build_info: &'static BuildInfo,
        config: SessionConfig,
        metrics: SessionMetrics,
    ) -> Session<T> {
        assert_ne!(config.conn_id, DUMMY_CONNECTION_ID);
        Self::new_internal(build_info, config, metrics)
    }

    /// Returns a reference-less collection of data usable by other tasks that don't have ownership
    /// of the Session.
    pub fn meta(&self) -> SessionMeta {
        SessionMeta {
            conn_id: self.conn_id().clone(),
            client_ip: self.client_ip().copied(),
            pcx: self.pcx().clone(),
            role_metadata: self.role_metadata().clone(),
            vars: self.vars.clone(),
        }

        // TODO: soft_assert that these are the same as Session.
    }

    /// Creates new statement logging metadata for a one-off
    /// statement.
    // Normally, such logging information would be created as part of
    // allocating a new prepared statement, and a refcounted handle
    // would be copied from that prepared statement to portals during
    // binding. However, we also support (via `Command::declare`)
    // binding a statement directly to a portal without creating an
    // intermediate prepared statement. Thus, for those cases, a
    // mechanism for generating the logging metadata directly is needed.
    pub(crate) fn mint_logging<A: AstInfo>(
        &self,
        raw_sql: String,
        stmt: Option<&Statement<A>>,
        now: EpochMillis,
    ) -> Arc<QCell<PreparedStatementLoggingInfo>> {
        Arc::new(QCell::new(
            &self.qcell_owner,
            PreparedStatementLoggingInfo::still_to_log(
                raw_sql,
                stmt,
                now,
                "".to_string(),
                self.uuid,
                false,
            ),
        ))
    }

    pub(crate) fn qcell_rw<'a, T2: 'a>(&'a mut self, cell: &'a Arc<QCell<T2>>) -> &'a mut T2 {
        self.qcell_owner.rw(&*cell)
    }

    /// Returns a unique ID for the session.
    /// Not to be confused with `connection_id`, which can be reused.
    pub fn uuid(&self) -> Uuid {
        self.uuid
    }

    /// Creates a new dummy session.
    ///
    /// Dummy sessions are intended for use when executing queries on behalf of
    /// the system itself, rather than on behalf of a user.
    pub fn dummy() -> Session<T> {
        let registry = MetricsRegistry::new();
        let metrics = Metrics::register_into(&registry);
        let metrics = metrics.session_metrics();
        let mut dummy = Self::new_internal(
            &DUMMY_BUILD_INFO,
            SessionConfig {
                conn_id: DUMMY_CONNECTION_ID,
                uuid: Uuid::new_v4(),
                user: SYSTEM_USER.name.clone(),
                client_ip: None,
                external_metadata_rx: None,
                internal_user_metadata: None,
                helm_chart_version: None,
            },
            metrics,
        );
        dummy.initialize_role_metadata(RoleId::User(0));
        dummy
    }

    fn new_internal(
        build_info: &'static BuildInfo,
        SessionConfig {
            conn_id,
            uuid,
            user,
            client_ip,
            mut external_metadata_rx,
            internal_user_metadata,
            helm_chart_version,
        }: SessionConfig,
        metrics: SessionMetrics,
    ) -> Session<T> {
        let (notices_tx, notices_rx) = mpsc::unbounded_channel();
        let default_cluster = INTERNAL_USER_NAME_TO_DEFAULT_CLUSTER.get(&user);
        let user = User {
            name: user,
            internal_metadata: internal_user_metadata,
            external_metadata: external_metadata_rx
                .as_mut()
                .map(|rx| rx.borrow_and_update().clone()),
        };
        let mut vars = SessionVars::new_unchecked(build_info, user, helm_chart_version);
        if let Some(default_cluster) = default_cluster {
            vars.set_cluster(default_cluster.clone());
        }
        Session {
            conn_id,
            uuid,
            transaction: TransactionStatus::Default,
            pcx: None,
            metrics,
            builtin_updates: None,
            prepared_statements: BTreeMap::new(),
            portals: BTreeMap::new(),
            role_metadata: None,
            client_ip,
            vars,
            notices_tx,
            notices_rx,
            next_transaction_id: 0,
            secret_key: rand::thread_rng().r#gen(),
            external_metadata_rx,
            qcell_owner: QCellOwner::new(),
            session_oracles: BTreeMap::new(),
        }
    }

    /// Returns the secret key associated with the session.
    pub fn secret_key(&self) -> u32 {
        self.secret_key
    }

    fn new_pcx(&self, mut wall_time: DateTime<Utc>) -> PlanContext {
        if let Some(mock_time) = self.vars().unsafe_new_transaction_wall_time() {
            wall_time = *mock_time;
        }
        PlanContext::new(wall_time)
    }

    /// Starts an explicit transaction, or changes an implicit to an explicit
    /// transaction.
    pub fn start_transaction(
        &mut self,
        wall_time: DateTime<Utc>,
        access: Option<TransactionAccessMode>,
        isolation_level: Option<TransactionIsolationLevel>,
    ) -> Result<(), AdapterError> {
        // Check that current transaction state is compatible with new `access`
        if let Some(txn) = self.transaction.inner() {
            // `READ WRITE` prohibited if:
            // - Currently in `READ ONLY`
            // - Already performed a query
            let read_write_prohibited = match txn.ops {
                TransactionOps::Peeks { .. } | TransactionOps::Subscribe => {
                    txn.access == Some(TransactionAccessMode::ReadOnly)
                }
                TransactionOps::None
                | TransactionOps::Writes(_)
                | TransactionOps::SingleStatement { .. }
                | TransactionOps::DDL { .. } => false,
            };

            if read_write_prohibited && access == Some(TransactionAccessMode::ReadWrite) {
                return Err(AdapterError::ReadWriteUnavailable);
            }
        }

        match std::mem::take(&mut self.transaction) {
            TransactionStatus::Default => {
                let id = self.next_transaction_id;
                self.next_transaction_id = self.next_transaction_id.wrapping_add(1);
                self.transaction = TransactionStatus::InTransaction(Transaction {
                    pcx: self.new_pcx(wall_time),
                    ops: TransactionOps::None,
                    write_lock_guards: None,
                    access,
                    id,
                });
            }
            TransactionStatus::Started(mut txn)
            | TransactionStatus::InTransactionImplicit(mut txn)
            | TransactionStatus::InTransaction(mut txn) => {
                if access.is_some() {
                    txn.access = access;
                }
                self.transaction = TransactionStatus::InTransaction(txn);
            }
            TransactionStatus::Failed(_) => unreachable!(),
        };

        if let Some(isolation_level) = isolation_level {
            self.vars
                .set_local_transaction_isolation(isolation_level.into());
        }

        Ok(())
    }

    /// Starts either a single statement or implicit transaction based on the
    /// number of statements, but only if no transaction has been started already.
    pub fn start_transaction_implicit(&mut self, wall_time: DateTime<Utc>, stmts: usize) {
        if let TransactionStatus::Default = self.transaction {
            let id = self.next_transaction_id;
            self.next_transaction_id = self.next_transaction_id.wrapping_add(1);
            let txn = Transaction {
                pcx: self.new_pcx(wall_time),
                ops: TransactionOps::None,
                write_lock_guards: None,
                access: None,
                id,
            };
            match stmts {
                1 => self.transaction = TransactionStatus::Started(txn),
                n if n > 1 => self.transaction = TransactionStatus::InTransactionImplicit(txn),
                _ => {}
            }
        }
    }

    /// Starts a single statement transaction, but only if no transaction has been started already.
    pub fn start_transaction_single_stmt(&mut self, wall_time: DateTime<Utc>) {
        self.start_transaction_implicit(wall_time, 1);
    }

    /// Clears a transaction, setting its state to Default and destroying all
    /// portals. Returned are:
    /// - sinks that were started in this transaction and need to be dropped
    /// - the cleared transaction so its operations can be handled
    ///
    /// The [Postgres protocol docs](https://www.postgresql.org/docs/current/protocol-flow.html#PROTOCOL-FLOW-EXT-QUERY) specify:
    /// > a named portal object lasts till the end of the current transaction
    /// and
    /// > An unnamed portal is destroyed at the end of the transaction
    #[must_use]
    pub fn clear_transaction(&mut self) -> TransactionStatus<T> {
        self.portals.clear();
        self.pcx = None;
        mem::take(&mut self.transaction)
    }

    /// Marks the current transaction as failed.
    pub fn fail_transaction(mut self) -> Self {
        match self.transaction {
            TransactionStatus::Default => unreachable!(),
            TransactionStatus::Started(txn)
            | TransactionStatus::InTransactionImplicit(txn)
            | TransactionStatus::InTransaction(txn) => {
                self.transaction = TransactionStatus::Failed(txn);
            }
            TransactionStatus::Failed(_) => {}
        };
        self
    }

    /// Returns the current transaction status.
    pub fn transaction(&self) -> &TransactionStatus<T> {
        &self.transaction
    }

    /// Returns the current transaction status.
    pub fn transaction_mut(&mut self) -> &mut TransactionStatus<T> {
        &mut self.transaction
    }

    /// Returns the session's transaction code.
    pub fn transaction_code(&self) -> TransactionCode {
        self.transaction().into()
    }

    /// Adds operations to the current transaction. An error is produced if
    /// they cannot be merged (i.e., a timestamp-dependent read cannot be
    /// merged to an insert).
    pub fn add_transaction_ops(&mut self, add_ops: TransactionOps<T>) -> Result<(), AdapterError> {
        self.transaction.add_ops(add_ops)
    }

    /// Returns a channel on which to send notices to the session.
    pub fn retain_notice_transmitter(&self) -> UnboundedSender<AdapterNotice> {
        self.notices_tx.clone()
    }

    /// Adds a notice to the session.
    pub fn add_notice(&self, notice: AdapterNotice) {
        self.add_notices([notice])
    }

    /// Adds multiple notices to the session.
    pub fn add_notices(&self, notices: impl IntoIterator<Item = AdapterNotice>) {
        for notice in notices {
            let _ = self.notices_tx.send(notice);
        }
    }

    /// Awaits a possible notice.
    ///
    /// This method is cancel safe.
    pub async fn recv_notice(&mut self) -> AdapterNotice {
        // This method is cancel safe because recv is cancel safe.
        loop {
            let notice = self
                .notices_rx
                .recv()
                .await
                .expect("Session also holds a sender, so recv won't ever return None");
            match self.notice_filter(notice) {
                Some(notice) => return notice,
                None => continue,
            }
        }
    }

    /// Returns a draining iterator over the notices attached to the session.
    pub fn drain_notices(&mut self) -> Vec<AdapterNotice> {
        let mut notices = Vec::new();
        while let Ok(notice) = self.notices_rx.try_recv() {
            if let Some(notice) = self.notice_filter(notice) {
                notices.push(notice);
            }
        }
        notices
    }

    /// Returns Some if the notice should be reported, otherwise None.
    fn notice_filter(&self, notice: AdapterNotice) -> Option<AdapterNotice> {
        // Filter out low threshold severity.
        let minimum_client_severity = self.vars.client_min_messages();
        let sev = notice.severity();
        if !minimum_client_severity.should_output_to_client(&sev) {
            return None;
        }
        // Filter out notices for other clusters.
        if let AdapterNotice::ClusterReplicaStatusChanged { cluster, .. } = &notice {
            if cluster != self.vars.cluster() {
                return None;
            }
        }
        Some(notice)
    }

    /// Sets the transaction ops to `TransactionOps::None`. Must only be used after
    /// verifying that no transaction anomalies will occur if cleared.
    pub fn clear_transaction_ops(&mut self) {
        if let Some(txn) = self.transaction.inner_mut() {
            txn.ops = TransactionOps::None;
        }
    }

    /// If the current transaction ops belong to a read, then sets the
    /// ops to `None`, returning the old read timestamp context if
    /// any existed. Must only be used after verifying that no transaction
    /// anomalies will occur if cleared.
    pub fn take_transaction_timestamp_context(&mut self) -> Option<TimestampContext<T>> {
        if let Some(Transaction { ops, .. }) = self.transaction.inner_mut() {
            if let TransactionOps::Peeks { .. } = ops {
                let ops = std::mem::take(ops);
                Some(
                    ops.timestamp_determination()
                        .expect("checked above")
                        .timestamp_context,
                )
            } else {
                None
            }
        } else {
            None
        }
    }

    /// Returns the transaction's read timestamp determination, if set.
    ///
    /// Returns `None` if there is no active transaction, or if the active
    /// transaction is not a read transaction.
    pub fn get_transaction_timestamp_determination(&self) -> Option<TimestampDetermination<T>> {
        match self.transaction.inner() {
            Some(Transaction {
                pcx: _,
                ops: TransactionOps::Peeks { determination, .. },
                write_lock_guards: _,
                access: _,
                id: _,
            }) => Some(determination.clone()),
            _ => None,
        }
    }

    /// Whether this session has a timestamp for a read transaction.
    pub fn contains_read_timestamp(&self) -> bool {
        matches!(
            self.transaction.inner(),
            Some(Transaction {
                pcx: _,
                ops: TransactionOps::Peeks {
                    determination: TimestampDetermination {
                        timestamp_context: TimestampContext::TimelineTimestamp { .. },
                        ..
                    },
                    ..
                },
                write_lock_guards: _,
                access: _,
                id: _,
            })
        )
    }

    /// Registers the prepared statement under `name`.
    pub fn set_prepared_statement(
        &mut self,
        name: String,
        stmt: Option<Statement<Raw>>,
        raw_sql: String,
        desc: StatementDesc,
        catalog_revision: u64,
        now: EpochMillis,
    ) {
        let logging = PreparedStatementLoggingInfo::still_to_log(
            raw_sql,
            stmt.as_ref(),
            now,
            name.clone(),
            self.uuid,
            false,
        );
        let statement = PreparedStatement {
            stmt,
            desc,
            catalog_revision,
            logging: Arc::new(QCell::new(&self.qcell_owner, logging)),
        };
        self.prepared_statements.insert(name, statement);
    }

    /// Removes the prepared statement associated with `name`.
    ///
    /// Returns whether a statement previously existed.
    pub fn remove_prepared_statement(&mut self, name: &str) -> bool {
        self.prepared_statements.remove(name).is_some()
    }

    /// Removes all prepared statements.
    pub fn remove_all_prepared_statements(&mut self) {
        self.prepared_statements.clear();
    }

    /// Retrieves the prepared statement associated with `name`.
    ///
    /// This is unverified and could be incorrect if the underlying catalog has
    /// changed.
    pub fn get_prepared_statement_unverified(&self, name: &str) -> Option<&PreparedStatement> {
        self.prepared_statements.get(name)
    }

    /// Retrieves the prepared statement associated with `name`.
    ///
    /// This is unverified and could be incorrect if the underlying catalog has
    /// changed.
    pub fn get_prepared_statement_mut_unverified(
        &mut self,
        name: &str,
    ) -> Option<&mut PreparedStatement> {
        self.prepared_statements.get_mut(name)
    }

    /// Returns the prepared statements for the session.
    pub fn prepared_statements(&self) -> &BTreeMap<String, PreparedStatement> {
        &self.prepared_statements
    }

    /// Binds the specified portal to the specified prepared statement.
    ///
    /// If the prepared statement contains parameters, the values and types of
    /// those parameters must be provided in `params`. It is the caller's
    /// responsibility to ensure that the correct number of parameters is
    /// provided.
    ///
    /// The `results_formats` parameter sets the desired format of the results,
    /// and is stored on the portal.
    pub fn set_portal(
        &mut self,
        portal_name: String,
        desc: StatementDesc,
        stmt: Option<Statement<Raw>>,
        logging: Arc<QCell<PreparedStatementLoggingInfo>>,
        params: Vec<(Datum, ScalarType)>,
        result_formats: Vec<Format>,
        catalog_revision: u64,
    ) -> Result<(), AdapterError> {
        // The empty portal can be silently replaced.
        if !portal_name.is_empty() && self.portals.contains_key(&portal_name) {
            return Err(AdapterError::DuplicateCursor(portal_name));
        }
        self.portals.insert(
            portal_name,
            Portal {
                stmt: stmt.map(Arc::new),
                desc,
                catalog_revision,
                parameters: Params {
                    datums: Row::pack(params.iter().map(|(d, _t)| d)),
                    types: params.into_iter().map(|(_d, t)| t).collect(),
                },
                result_formats,
                state: PortalState::NotStarted,
                logging,
            },
        );
        Ok(())
    }

    /// Removes the specified portal.
    ///
    /// If there is no such portal, this method does nothing. Returns whether that portal existed.
    pub fn remove_portal(&mut self, portal_name: &str) -> bool {
        self.portals.remove(portal_name).is_some()
    }

    /// Retrieves a reference to the specified portal.
    ///
    /// If there is no such portal, returns `None`.
    pub fn get_portal_unverified(&self, portal_name: &str) -> Option<&Portal> {
        self.portals.get(portal_name)
    }

    /// Retrieves a mutable reference to the specified portal.
    ///
    /// If there is no such portal, returns `None`.
    pub fn get_portal_unverified_mut(&mut self, portal_name: &str) -> Option<&mut Portal> {
        self.portals.get_mut(portal_name)
    }

    /// Creates and installs a new portal.
    pub fn create_new_portal(
        &mut self,
        stmt: Option<Statement<Raw>>,
        logging: Arc<QCell<PreparedStatementLoggingInfo>>,
        desc: StatementDesc,
        parameters: Params,
        result_formats: Vec<Format>,
        catalog_revision: u64,
    ) -> Result<String, AdapterError> {
        // See: https://github.com/postgres/postgres/blob/84f5c2908dad81e8622b0406beea580e40bb03ac/src/backend/utils/mmgr/portalmem.c#L234

        for i in 0usize.. {
            let name = format!("<unnamed portal {}>", i);
            match self.portals.entry(name.clone()) {
                Entry::Occupied(_) => continue,
                Entry::Vacant(entry) => {
                    entry.insert(Portal {
                        stmt: stmt.map(Arc::new),
                        desc,
                        catalog_revision,
                        parameters,
                        result_formats,
                        state: PortalState::NotStarted,
                        logging,
                    });
                    return Ok(name);
                }
            }
        }

        coord_bail!("unable to create a new portal");
    }

    /// Resets the session to its initial state. Returns sinks that need to be
    /// dropped.
    pub fn reset(&mut self) {
        let _ = self.clear_transaction();
        self.prepared_statements.clear();
        self.vars.reset_all();
    }

    /// Returns the [application_name] that created this session.
    ///
    /// [application_name]: (https://www.postgresql.org/docs/current/runtime-config-logging.html#GUC-APPLICATION-NAME)
    pub fn application_name(&self) -> &str {
        self.vars.application_name()
    }

    /// Returns a reference to the variables in this session.
    pub fn vars(&self) -> &SessionVars {
        &self.vars
    }

    /// Returns a mutable reference to the variables in this session.
    pub fn vars_mut(&mut self) -> &mut SessionVars {
        &mut self.vars
    }

    /// Grants a set of write locks to this session's inner [`Transaction`].
    ///
    /// # Panics
    /// If the inner transaction is idle. See [`TransactionStatus::try_grant_write_locks`].
    ///
    pub fn try_grant_write_locks(&mut self, guards: WriteLocks) -> Result<(), &WriteLocks> {
        self.transaction.try_grant_write_locks(guards)
    }

    /// Drains any external metadata updates and applies the changes from the latest update.
    pub fn apply_external_metadata_updates(&mut self) {
        // If no sender is registered then there isn't anything to do.
        let Some(rx) = &mut self.external_metadata_rx else {
            return;
        };

        // If the value hasn't changed then return.
        if !rx.has_changed().unwrap_or(false) {
            return;
        }

        // Update our metadata! Note the short critical section (just a clone) to avoid blocking
        // the sending side of this watch channel.
        let metadata = rx.borrow_and_update().clone();
        self.vars.set_external_user_metadata(metadata);
    }

    /// Initializes the session's role metadata.
    pub fn initialize_role_metadata(&mut self, role_id: RoleId) {
        self.role_metadata = Some(RoleMetadata::new(role_id));
    }

    /// Ensures that a timestamp oracle exists for `timeline` and returns a mutable reference to
    /// the timestamp oracle.
    pub fn ensure_timestamp_oracle(
        &mut self,
        timeline: Timeline,
    ) -> &mut InMemoryTimestampOracle<T, NowFn<T>> {
        self.session_oracles
            .entry(timeline)
            .or_insert_with(|| InMemoryTimestampOracle::new(T::minimum(), NowFn::from(T::minimum)))
    }

    /// Ensures that a timestamp oracle exists for reads and writes from/to a local input and
    /// returns a mutable reference to the timestamp oracle.
    pub fn ensure_local_timestamp_oracle(&mut self) -> &mut InMemoryTimestampOracle<T, NowFn<T>> {
        self.ensure_timestamp_oracle(Timeline::EpochMilliseconds)
    }

    /// Returns a reference to the timestamp oracle for `timeline`.
    pub fn get_timestamp_oracle(
        &self,
        timeline: &Timeline,
    ) -> Option<&InMemoryTimestampOracle<T, NowFn<T>>> {
        self.session_oracles.get(timeline)
    }

    /// If the current session is using the Strong Session Serializable isolation level advance the
    /// session local timestamp oracle to `write_ts`.
    pub fn apply_write(&mut self, timestamp: T) {
        if self.vars().transaction_isolation() == &IsolationLevel::StrongSessionSerializable {
            self.ensure_local_timestamp_oracle().apply_write(timestamp);
        }
    }

    /// Returns the [`SessionMetrics`] instance associated with this [`Session`].
    pub fn metrics(&self) -> &SessionMetrics {
        &self.metrics
    }

    /// Sets the `BuiltinTableAppendNotify` for this session.
    pub fn set_builtin_table_updates(&mut self, fut: BuiltinTableAppendNotify) {
        let prev = self.builtin_updates.replace(fut);
        mz_ore::soft_assert_or_log!(prev.is_none(), "replacing old builtin table notify");
    }

    /// Takes the stashed `BuiltinTableAppendNotify`, if one exists, and returns a [`Future`] that
    /// waits for the writes to complete.
    pub fn clear_builtin_table_updates(&mut self) -> Option<impl Future<Output = ()> + 'static> {
        if let Some(fut) = self.builtin_updates.take() {
            // Record how long we blocked for, if we blocked at all.
            let histogram = self
                .metrics()
                .session_startup_table_writes_seconds()
                .clone();
            Some(async move {
                fut.wall_time().observe(histogram).await;
            })
        } else {
            None
        }
    }
}

/// A prepared statement.
#[derive(Derivative, Clone)]
#[derivative(Debug)]
pub struct PreparedStatement {
    stmt: Option<Statement<Raw>>,
    desc: StatementDesc,
    /// The most recent catalog revision that has verified this statement.
    pub catalog_revision: u64,
    #[derivative(Debug = "ignore")]
    logging: Arc<QCell<PreparedStatementLoggingInfo>>,
}

impl PreparedStatement {
    /// Returns the AST associated with this prepared statement,
    /// if the prepared statement was not the empty query.
    pub fn stmt(&self) -> Option<&Statement<Raw>> {
        self.stmt.as_ref()
    }

    /// Returns the description of the prepared statement.
    pub fn desc(&self) -> &StatementDesc {
        &self.desc
    }

    /// Returns a handle to the metadata for statement logging.
    pub fn logging(&self) -> &Arc<QCell<PreparedStatementLoggingInfo>> {
        &self.logging
    }
}

/// A portal represents the execution state of a running or runnable query.
#[derive(Derivative)]
#[derivative(Debug)]
pub struct Portal {
    /// The statement that is bound to this portal.
    pub stmt: Option<Arc<Statement<Raw>>>,
    /// The statement description.
    pub desc: StatementDesc,
    /// The most recent catalog revision that has verified this statement.
    pub catalog_revision: u64,
    /// The bound values for the parameters in the prepared statement, if any.
    pub parameters: Params,
    /// The desired output format for each column in the result set.
    pub result_formats: Vec<Format>,
    /// A handle to metadata needed for statement logging.
    #[derivative(Debug = "ignore")]
    pub logging: Arc<QCell<PreparedStatementLoggingInfo>>,
    /// The execution state of the portal.
    #[derivative(Debug = "ignore")]
    pub state: PortalState,
}

/// Execution states of a portal.
pub enum PortalState {
    /// Portal not yet started.
    NotStarted,
    /// Portal is a rows-returning statement in progress with 0 or more rows
    /// remaining.
    InProgress(Option<InProgressRows>),
    /// Portal has completed and should not be re-executed. If the optional string
    /// is present, it is returned as a CommandComplete tag, otherwise an error
    /// is sent.
    Completed(Option<String>),
}

/// State of an in-progress, rows-returning portal.
pub struct InProgressRows {
    /// The current batch of rows.
    pub current: Option<Box<dyn RowIterator + Send + Sync>>,
    /// A stream from which to fetch more row batches.
    pub remaining: RecordFirstRowStream,
}

impl InProgressRows {
    /// Creates a new InProgressRows from a batch stream.
    pub fn new(remaining: RecordFirstRowStream) -> Self {
        Self {
            current: None,
            remaining,
        }
    }
}

/// A channel of batched rows.
pub type RowBatchStream = UnboundedReceiver<PeekResponseUnary>;

/// The transaction status of a session.
///
/// PostgreSQL's transaction states are in backend/access/transam/xact.c.
#[derive(Debug)]
pub enum TransactionStatus<T> {
    /// Idle. Matches `TBLOCK_DEFAULT`.
    Default,
    /// Running a single-query transaction. Matches
    /// `TBLOCK_STARTED`. In PostgreSQL, when using the extended query protocol, this
    /// may be upgraded into multi-statement implicit query (see [`Self::InTransactionImplicit`]).
    /// Additionally, some statements may trigger an eager commit of the implicit transaction,
    /// see: <https://git.postgresql.org/gitweb/?p=postgresql.git&a=commitdiff&h=f92944137>. In
    /// Materialize however, we eagerly commit all statements outside of an explicit transaction
    /// when using the extended query protocol. Therefore, we can guarantee that this state will
    /// always be a single-query transaction and never be upgraded into a multi-statement implicit
    /// query.
    Started(Transaction<T>),
    /// Currently in a transaction issued from a `BEGIN`. Matches `TBLOCK_INPROGRESS`.
    InTransaction(Transaction<T>),
    /// Currently in an implicit transaction started from a multi-statement query
    /// with more than 1 statements. Matches `TBLOCK_IMPLICIT_INPROGRESS`.
    InTransactionImplicit(Transaction<T>),
    /// In a failed transaction. Matches `TBLOCK_ABORT`.
    Failed(Transaction<T>),
}

impl<T: TimestampManipulation> TransactionStatus<T> {
    /// Extracts the inner transaction ops and write lock guard if not failed.
    pub fn into_ops_and_lock_guard(self) -> (Option<TransactionOps<T>>, Option<WriteLocks>) {
        match self {
            TransactionStatus::Default | TransactionStatus::Failed(_) => (None, None),
            TransactionStatus::Started(txn)
            | TransactionStatus::InTransaction(txn)
            | TransactionStatus::InTransactionImplicit(txn) => {
                (Some(txn.ops), txn.write_lock_guards)
            }
        }
    }

    /// Exposes the inner transaction.
    pub fn inner(&self) -> Option<&Transaction<T>> {
        match self {
            TransactionStatus::Default => None,
            TransactionStatus::Started(txn)
            | TransactionStatus::InTransaction(txn)
            | TransactionStatus::InTransactionImplicit(txn)
            | TransactionStatus::Failed(txn) => Some(txn),
        }
    }

    /// Exposes the inner transaction.
    pub fn inner_mut(&mut self) -> Option<&mut Transaction<T>> {
        match self {
            TransactionStatus::Default => None,
            TransactionStatus::Started(txn)
            | TransactionStatus::InTransaction(txn)
            | TransactionStatus::InTransactionImplicit(txn)
            | TransactionStatus::Failed(txn) => Some(txn),
        }
    }

    /// Whether the transaction's ops are DDL.
    pub fn is_ddl(&self) -> bool {
        match self {
            TransactionStatus::Default => false,
            TransactionStatus::Started(txn)
            | TransactionStatus::InTransaction(txn)
            | TransactionStatus::InTransactionImplicit(txn)
            | TransactionStatus::Failed(txn) => {
                matches!(txn.ops, TransactionOps::DDL { .. })
            }
        }
    }

    /// Expresses whether or not the transaction was implicitly started.
    /// However, its negation does not imply explicitly started.
    pub fn is_implicit(&self) -> bool {
        match self {
            TransactionStatus::Started(_) | TransactionStatus::InTransactionImplicit(_) => true,
            TransactionStatus::Default
            | TransactionStatus::InTransaction(_)
            | TransactionStatus::Failed(_) => false,
        }
    }

    /// Whether the transaction may contain multiple statements.
    pub fn is_in_multi_statement_transaction(&self) -> bool {
        match self {
            TransactionStatus::InTransaction(_) | TransactionStatus::InTransactionImplicit(_) => {
                true
            }
            TransactionStatus::Default
            | TransactionStatus::Started(_)
            | TransactionStatus::Failed(_) => false,
        }
    }

    /// Whether the transaction is in a multi-statement, immediate transaction.
    pub fn in_immediate_multi_stmt_txn(&self, when: &QueryWhen) -> bool {
        self.is_in_multi_statement_transaction() && when == &QueryWhen::Immediately
    }

    /// Grants the writes lock to the inner transaction, returning an error if the transaction
    /// has already been granted write locks.
    ///
    /// # Panics
    /// If `self` is `TransactionStatus::Default`, which indicates that the
    /// transaction is idle, which is not appropriate to assign the
    /// coordinator's write lock to.
    ///
    pub fn try_grant_write_locks(&mut self, guards: WriteLocks) -> Result<(), &WriteLocks> {
        match self {
            TransactionStatus::Default => panic!("cannot grant write lock to txn not yet started"),
            TransactionStatus::Started(txn)
            | TransactionStatus::InTransaction(txn)
            | TransactionStatus::InTransactionImplicit(txn)
            | TransactionStatus::Failed(txn) => txn.try_grant_write_locks(guards),
        }
    }

    /// Returns the currently held [`WriteLocks`], if this transaction holds any.
    pub fn write_locks(&self) -> Option<&WriteLocks> {
        match self {
            TransactionStatus::Default => None,
            TransactionStatus::Started(txn)
            | TransactionStatus::InTransaction(txn)
            | TransactionStatus::InTransactionImplicit(txn)
            | TransactionStatus::Failed(txn) => txn.write_lock_guards.as_ref(),
        }
    }

    /// The timeline of the transaction, if one exists.
    pub fn timeline(&self) -> Option<Timeline> {
        match self {
            TransactionStatus::Default => None,
            TransactionStatus::Started(txn)
            | TransactionStatus::InTransaction(txn)
            | TransactionStatus::InTransactionImplicit(txn)
            | TransactionStatus::Failed(txn) => txn.timeline(),
        }
    }

    /// The cluster of the transaction, if one exists.
    pub fn cluster(&self) -> Option<ClusterId> {
        match self {
            TransactionStatus::Default => None,
            TransactionStatus::Started(txn)
            | TransactionStatus::InTransaction(txn)
            | TransactionStatus::InTransactionImplicit(txn)
            | TransactionStatus::Failed(txn) => txn.cluster(),
        }
    }

    /// Snapshot of the catalog that reflects DDL operations run in this transaction.
    pub fn catalog_state(&self) -> Option<&CatalogState> {
        match self.inner() {
            Some(Transaction {
                ops: TransactionOps::DDL { state, .. },
                ..
            }) => Some(state),
            _ => None,
        }
    }

    /// Reports whether any operations have been executed as part of this transaction
    pub fn contains_ops(&self) -> bool {
        match self.inner() {
            Some(txn) => txn.contains_ops(),
            None => false,
        }
    }

    /// Adds operations to the current transaction. An error is produced if they cannot be merged
    /// (i.e., a timestamp-dependent read cannot be merged to an insert).
    ///
    /// The `DDL` variant is an exception and does not merge operations, but instead overwrites the
    /// old ops with the new ops. This is correct because it is only used in conjunction with the
    /// Dry Run catalog op which returns an error containing all of the ops, and those ops are
    /// passed to this function which then overwrites.
    ///
    /// # Panics
    /// If the operations are compatible but the operation metadata doesn't match. Such as reads at
    /// different timestamps, reads on different timelines, reads on different clusters, etc. It's
    /// up to the caller to make sure these are aligned.
    pub fn add_ops(&mut self, add_ops: TransactionOps<T>) -> Result<(), AdapterError> {
        match self {
            TransactionStatus::Started(Transaction { ops, access, .. })
            | TransactionStatus::InTransaction(Transaction { ops, access, .. })
            | TransactionStatus::InTransactionImplicit(Transaction { ops, access, .. }) => {
                match ops {
                    TransactionOps::None => {
                        if matches!(access, Some(TransactionAccessMode::ReadOnly))
                            && matches!(add_ops, TransactionOps::Writes(_))
                        {
                            return Err(AdapterError::ReadOnlyTransaction);
                        }
                        *ops = add_ops;
                    }
                    TransactionOps::Peeks {
                        determination,
                        cluster_id,
                        requires_linearization,
                    } => match add_ops {
                        TransactionOps::Peeks {
                            determination: add_timestamp_determination,
                            cluster_id: add_cluster_id,
                            requires_linearization: add_requires_linearization,
                        } => {
                            assert_eq!(*cluster_id, add_cluster_id);
                            match (
                                &determination.timestamp_context,
                                &add_timestamp_determination.timestamp_context,
                            ) {
                                (
                                    TimestampContext::TimelineTimestamp {
                                        timeline: txn_timeline,
                                        chosen_ts: txn_ts,
                                        oracle_ts: _,
                                    },
                                    TimestampContext::TimelineTimestamp {
                                        timeline: add_timeline,
                                        chosen_ts: add_ts,
                                        oracle_ts: _,
                                    },
                                ) => {
                                    assert_eq!(txn_timeline, add_timeline);
                                    assert_eq!(txn_ts, add_ts);
                                }
                                (TimestampContext::NoTimestamp, _) => {
                                    *determination = add_timestamp_determination
                                }
                                (_, TimestampContext::NoTimestamp) => {}
                            };
                            if matches!(requires_linearization, RequireLinearization::NotRequired)
                                && matches!(
                                    add_requires_linearization,
                                    RequireLinearization::Required
                                )
                            {
                                *requires_linearization = add_requires_linearization;
                            }
                        }
                        // Iff peeks thus far do not have a timestamp (i.e.
                        // they are constant), we can switch to a write
                        // transaction.
                        writes @ TransactionOps::Writes(..)
                            if !determination.timestamp_context.contains_timestamp() =>
                        {
                            *ops = writes;
                        }
                        _ => return Err(AdapterError::ReadOnlyTransaction),
                    },
                    TransactionOps::Subscribe => {
                        return Err(AdapterError::SubscribeOnlyTransaction);
                    }
                    TransactionOps::Writes(txn_writes) => match add_ops {
                        TransactionOps::Writes(mut add_writes) => {
                            // We should have already checked the access above, but make sure we don't miss
                            // it anyway.
                            assert!(!matches!(access, Some(TransactionAccessMode::ReadOnly)));
                            txn_writes.append(&mut add_writes);
                        }
                        // Iff peeks do not have a timestamp (i.e. they are
                        // constant), we can permit them.
                        TransactionOps::Peeks { determination, .. }
                            if !determination.timestamp_context.contains_timestamp() => {}
                        _ => {
                            return Err(AdapterError::WriteOnlyTransaction);
                        }
                    },
                    TransactionOps::SingleStatement { .. } => {
                        return Err(AdapterError::SingleStatementTransaction);
                    }
                    TransactionOps::DDL {
                        ops: og_ops,
                        revision: og_revision,
                        state: og_state,
                    } => match add_ops {
                        TransactionOps::DDL {
                            ops: new_ops,
                            revision: new_revision,
                            state: new_state,
                        } => {
                            if *og_revision != new_revision {
                                return Err(AdapterError::DDLTransactionRace);
                            }
                            // The old og_ops are overwritten, not extended.
                            if !new_ops.is_empty() {
                                *og_ops = new_ops;
                                *og_state = new_state;
                            }
                        }
                        _ => return Err(AdapterError::DDLOnlyTransaction),
                    },
                }
            }
            TransactionStatus::Default | TransactionStatus::Failed(_) => {
                unreachable!()
            }
        }
        Ok(())
    }
}

/// An abstraction allowing us to identify different transactions.
pub type TransactionId = u64;

impl<T> Default for TransactionStatus<T> {
    fn default() -> Self {
        TransactionStatus::Default
    }
}

/// State data for transactions.
#[derive(Debug)]
pub struct Transaction<T> {
    /// Plan context.
    pub pcx: PlanContext,
    /// Transaction operations.
    pub ops: TransactionOps<T>,
    /// Uniquely identifies the transaction on a per connection basis.
    /// Two transactions started from separate connections may share the
    /// same ID.
    /// If all IDs have been exhausted, this will wrap around back to 0.
    pub id: TransactionId,
    /// Locks for objects this transaction will operate on.
    write_lock_guards: Option<WriteLocks>,
    /// Access mode (read only, read write).
    access: Option<TransactionAccessMode>,
}

impl<T> Transaction<T> {
    /// Tries to grant the write lock to this transaction for the remainder of its lifetime. Errors
    /// if this [`Transaction`] has already been granted write locks.
    fn try_grant_write_locks(&mut self, guards: WriteLocks) -> Result<(), &WriteLocks> {
        match &mut self.write_lock_guards {
            Some(existing) => Err(existing),
            locks @ None => {
                *locks = Some(guards);
                Ok(())
            }
        }
    }

    /// The timeline of the transaction, if one exists.
    fn timeline(&self) -> Option<Timeline> {
        match &self.ops {
            TransactionOps::Peeks {
                determination:
                    TimestampDetermination {
                        timestamp_context: TimestampContext::TimelineTimestamp { timeline, .. },
                        ..
                    },
                ..
            } => Some(timeline.clone()),
            TransactionOps::Peeks { .. }
            | TransactionOps::None
            | TransactionOps::Subscribe
            | TransactionOps::Writes(_)
            | TransactionOps::SingleStatement { .. }
            | TransactionOps::DDL { .. } => None,
        }
    }

    /// The cluster of the transaction, if one exists.
    pub fn cluster(&self) -> Option<ClusterId> {
        match &self.ops {
            TransactionOps::Peeks { cluster_id, .. } => Some(cluster_id.clone()),
            TransactionOps::None
            | TransactionOps::Subscribe
            | TransactionOps::Writes(_)
            | TransactionOps::SingleStatement { .. }
            | TransactionOps::DDL { .. } => None,
        }
    }

    /// Reports whether any operations have been executed as part of this transaction
    fn contains_ops(&self) -> bool {
        !matches!(self.ops, TransactionOps::None)
    }
}

/// A transaction's status code.
#[derive(Debug, Clone, Copy)]
pub enum TransactionCode {
    /// Not currently in a transaction
    Idle,
    /// Currently in a transaction
    InTransaction,
    /// Currently in a transaction block which is failed
    Failed,
}

impl From<TransactionCode> for u8 {
    fn from(code: TransactionCode) -> Self {
        match code {
            TransactionCode::Idle => b'I',
            TransactionCode::InTransaction => b'T',
            TransactionCode::Failed => b'E',
        }
    }
}

impl From<TransactionCode> for String {
    fn from(code: TransactionCode) -> Self {
        char::from(u8::from(code)).to_string()
    }
}

impl<T> From<&TransactionStatus<T>> for TransactionCode {
    /// Convert from the Session's version
    fn from(status: &TransactionStatus<T>) -> TransactionCode {
        match status {
            TransactionStatus::Default => TransactionCode::Idle,
            TransactionStatus::Started(_) => TransactionCode::InTransaction,
            TransactionStatus::InTransaction(_) => TransactionCode::InTransaction,
            TransactionStatus::InTransactionImplicit(_) => TransactionCode::InTransaction,
            TransactionStatus::Failed(_) => TransactionCode::Failed,
        }
    }
}

/// The type of operation being performed by the transaction.
///
/// This is needed because we currently do not allow mixing reads and writes in
/// a transaction. Use this to record what we have done, and what may need to
/// happen at commit.
#[derive(Debug)]
pub enum TransactionOps<T> {
    /// The transaction has been initiated, but no statement has yet been executed
    /// in it.
    None,
    /// This transaction has had a peek (`SELECT`, `SUBSCRIBE`). If the inner value
    /// is has a timestamp, it must only do other peeks. However, if it doesn't
    /// have a timestamp (i.e. the values are constants), the transaction can still
    /// perform writes.
    Peeks {
        /// The timestamp and timestamp related metadata for the peek.
        determination: TimestampDetermination<T>,
        /// The cluster used to execute peeks.
        cluster_id: ClusterId,
        /// Whether this peek needs to be linearized.
        requires_linearization: RequireLinearization,
    },
    /// This transaction has done a `SUBSCRIBE` and must do nothing else.
    Subscribe,
    /// This transaction has had a write (`INSERT`, `UPDATE`, `DELETE`) and must
    /// only do other writes, or reads whose timestamp is None (i.e. constants).
    Writes(Vec<WriteOp>),
    /// This transaction has a prospective statement that will execute during commit.
    SingleStatement {
        /// The prospective statement.
        stmt: Arc<Statement<Raw>>,
        /// The statement params.
        params: mz_sql::plan::Params,
    },
    /// This transaction has run some _simple_ DDL and must do nothing else. Any statement/plan that
    /// uses this must return false in `must_serialize_ddl()` because this is serialized instead in
    /// `sequence_plan()` during `COMMIT`.
    DDL {
        /// Catalog operations that have already run, and must run before each subsequent op.
        ops: Vec<crate::catalog::Op>,
        /// In-memory state that reflects the previously applied ops.
        state: CatalogState,
        /// Transient revision of the `Catalog` when this transaction started.
        revision: u64,
    },
}

impl<T> TransactionOps<T> {
    fn timestamp_determination(self) -> Option<TimestampDetermination<T>> {
        match self {
            TransactionOps::Peeks { determination, .. } => Some(determination),
            TransactionOps::None
            | TransactionOps::Subscribe
            | TransactionOps::Writes(_)
            | TransactionOps::SingleStatement { .. }
            | TransactionOps::DDL { .. } => None,
        }
    }
}

impl<T> Default for TransactionOps<T> {
    fn default() -> Self {
        Self::None
    }
}

/// An `INSERT` waiting to be committed.
#[derive(Debug, Clone, PartialEq)]
pub struct WriteOp {
    /// The target table.
    pub id: CatalogItemId,
    /// The data rows.
    pub rows: TableData,
}

/// Whether a transaction requires linearization.
#[derive(Debug)]
pub enum RequireLinearization {
    /// Linearization is required.
    Required,
    /// Linearization is not required.
    NotRequired,
}

impl From<&ExplainContext> for RequireLinearization {
    fn from(ctx: &ExplainContext) -> Self {
        match ctx {
            ExplainContext::None | ExplainContext::PlanInsightsNotice(_) => {
                RequireLinearization::Required
            }
            _ => RequireLinearization::NotRequired,
        }
    }
}

/// A complete set of exclusive locks for writing to collections identified by [`CatalogItemId`]s.
///
/// To prevent deadlocks between two sessions, we do not allow acquiring a partial set of locks.
#[derive(Debug)]
pub struct WriteLocks {
    locks: BTreeMap<CatalogItemId, tokio::sync::OwnedMutexGuard<()>>,
    /// Connection that currently holds these locks, used for tracing purposes only.
    conn_id: ConnectionId,
}

impl WriteLocks {
    /// Create a [`WriteLocksBuilder`] pre-defining all of the locks we need.
    ///
    /// When "finishing" the builder with [`WriteLocksBuilder::all_or_nothing`], if we haven't
    /// acquired all of the necessary locks we drop any partially acquired ones.
    pub fn builder(sources: impl IntoIterator<Item = CatalogItemId>) -> WriteLocksBuilder {
        let locks = sources.into_iter().map(|gid| (gid, None)).collect();
        WriteLocksBuilder { locks }
    }

    /// Validate this set of [`WriteLocks`] is sufficient for the provided collections.
    /// Dropping the currently held locks if it's not.
    pub fn validate(
        self,
        collections: impl Iterator<Item = CatalogItemId>,
    ) -> Result<Self, BTreeSet<CatalogItemId>> {
        let mut missing = BTreeSet::new();
        for collection in collections {
            if !self.locks.contains_key(&collection) {
                missing.insert(collection);
            }
        }

        if missing.is_empty() {
            Ok(self)
        } else {
            // Explicitly drop the already acquired locks.
            drop(self);
            Err(missing)
        }
    }
}

impl Drop for WriteLocks {
    fn drop(&mut self) {
        // We may have merged the locks into GroupCommitWriteLocks, thus it could be empty.
        if !self.locks.is_empty() {
            tracing::info!(
                conn_id = %self.conn_id,
                locks = ?self.locks,
                "dropping write locks",
            );
        }
    }
}

/// A builder struct that helps us acquire all of the locks we need, or none of them.
///
/// See [`WriteLocks::builder`].
#[derive(Debug)]
pub struct WriteLocksBuilder {
    locks: BTreeMap<CatalogItemId, Option<tokio::sync::OwnedMutexGuard<()>>>,
}

impl WriteLocksBuilder {
    /// Adds a lock to this builder.
    pub fn insert_lock(&mut self, id: CatalogItemId, lock: tokio::sync::OwnedMutexGuard<()>) {
        self.locks.insert(id, Some(lock));
    }

    /// Finish this builder by returning either all of the necessary locks, or none of them.
    ///
    /// If we fail to acquire all of the locks, returns one of the [`CatalogItemId`]s that we
    /// failed to acquire a lock for, that should be awaited so we know when to run again.
    pub fn all_or_nothing(self, conn_id: &ConnectionId) -> Result<WriteLocks, CatalogItemId> {
        let (locks, missing): (BTreeMap<_, _>, BTreeSet<_>) =
            self.locks
                .into_iter()
                .partition_map(|(gid, lock)| match lock {
                    Some(lock) => itertools::Either::Left((gid, lock)),
                    None => itertools::Either::Right(gid),
                });

        match missing.iter().next() {
            None => {
                tracing::info!(%conn_id, ?locks, "acquired write locks");
                Ok(WriteLocks {
                    locks,
                    conn_id: conn_id.clone(),
                })
            }
            Some(gid) => {
                tracing::info!(?missing, "failed to acquire write locks");
                // Explicitly drop the already acquired locks.
                drop(locks);
                Err(*gid)
            }
        }
    }
}

/// Collection of [`WriteLocks`] gathered during [`group_commit`].
///
/// Note: This struct should __never__ be used outside of group commit because it attempts to merge
/// together several collections of [`WriteLocks`] which if not done carefully can cause deadlocks
/// or consistency violations.
///
/// We must prevent writes from occurring to tables during read then write plans (e.g. `UPDATE`)
/// but we can allow blind writes (e.g. `INSERT`) to get committed concurrently at the same
/// timestamp when submitting the updates from a read then write plan.
///
/// Naively it would seem as though we could allow blind writes to occur whenever as blind writes
/// could never cause invalid retractions, but it could cause us to violate serializability because
/// there is no total order we could define for the transactions. Consider the following scenario:
///
/// ```text
/// table: foo
///
///  a | b
/// --------
///  x   2
///  y   3
///  z   4
///
/// -- Session(A)
/// -- read then write plan, reads at t0, writes at t3, transaction Ta
/// DELETE FROM foo WHERE b % 2 = 0;
///
///
/// -- Session(B)
/// -- blind write into foo, writes at t1, transaction Tb
/// INSERT INTO foo VALUES ('q', 6);
/// -- select from foo, reads at t2, transaction Tc
/// SELECT * FROM foo;
///
///
/// The times these operations occur at are ordered:
/// t0 < t1 < t2 < t3
///
/// Given the timing of the operations, the transactions must have the following order:
///
/// * Ta does not observe ('q', 6), so Ta < Tb
/// * Tc does observe ('q', 6), so Tb < Tc
/// * Tc does not observe the retractions from Ta, so Tc < Ta
///
/// For total order to exist, Ta < Tb < Tc < Ta, which is impossible.
/// ```
///
/// [`group_commit`]: super::coord::Coordinator::group_commit
#[derive(Debug, Default)]
pub(crate) struct GroupCommitWriteLocks {
    locks: BTreeMap<CatalogItemId, tokio::sync::OwnedMutexGuard<()>>,
}

impl GroupCommitWriteLocks {
    /// Merge a set of [`WriteLocks`] into this collection for group commit.
    pub fn merge(&mut self, mut locks: WriteLocks) {
        // Note: Ideally we would use `.drain`, but that method doesn't exist for BTreeMap.
        //
        // See: <https://github.com/rust-lang/rust/issues/81074>
        let existing = std::mem::take(&mut locks.locks);
        self.locks.extend(existing);
    }

    /// Returns the collections we're missing locks for, if any.
    pub fn missing_locks(
        &self,
        writes: impl Iterator<Item = CatalogItemId>,
    ) -> BTreeSet<CatalogItemId> {
        let mut missing = BTreeSet::new();
        for write in writes {
            if !self.locks.contains_key(&write) {
                missing.insert(write);
            }
        }
        missing
    }
}

impl Drop for GroupCommitWriteLocks {
    fn drop(&mut self) {
        if !self.locks.is_empty() {
            tracing::info!(
                locks = ?self.locks,
                "dropping group commit write locks",
            );
        }
    }
}