mz_storage/source/postgres/

snapshot.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.

//! Renders the table snapshot side of the [`PostgresSourceConnection`] ingestion dataflow.
//!
//! # Snapshot reading
//!
//! Depending on the resumption LSNs the table reader decides which tables need to be snapshotted
//! and performs a simple `COPY` query on them in order to get a snapshot. There are a few subtle
//! points about this operation, described in the following sections.
//!
//! ## Consistent LSN point for snapshot transactions
//!
//! Given that all our ingestion is based on correctly timestamping updates with the LSN they
//! happened at it is important that we run the `COPY` query at a specific LSN point that is
//! relatable with the LSN numbers we receive from the replication stream. Such point does not
//! necessarily exist for a normal SQL transaction. To achieve this we must force postgres to
//! produce a consistent point and let us know of the LSN number of that by creating a replication
//! slot as the first statement in a transaction.
//!
//! This is a temporary dummy slot that is only used to put our snapshot transaction on a
//! consistent LSN point. Unfortunately no lighterweight method exists for doing this. See this
//! [postgres thread] for more details.
//!
//! One might wonder why we don't use the actual real slot to provide us with the snapshot point
//! which would automatically be at the correct LSN. The answer is that it's possible that we crash
//! and restart after having already created the slot but before having finished the snapshot. In
//! that case the restarting process will have lost its opportunity to run queries at the slot's
//! consistent point as that opportunity only exists in the ephemeral transaction that created the
//! slot and that is long gone. Additionally there are good reasons of why we'd like to move the
//! slot creation much earlier, e.g during purification, in which case the slot will always be
//! pre-created.
//!
//! [postgres thread]: https://www.postgresql.org/message-id/flat/CAMN0T-vzzNy6TV1Jvh4xzNQdAvCLBQK_kh6_U7kAXgGU3ZFg-Q%40mail.gmail.com
//!
//! ## Reusing the consistent point among all workers
//!
//! Creating replication slots is potentially expensive so the code makes is such that all workers
//! cooperate and reuse one consistent snapshot among them. In order to do so we make use the
//! "export transaction" feature of postgres. This feature allows one SQL session to create an
//! identifier for the transaction (a string identifier) it is currently in, which can be used by
//! other sessions to enter the same "snapshot".
//!
//! We accomplish this by picking one worker at random to function as the transaction leader. The
//! transaction leader is responsible for starting a SQL session, creating a temporary replication
//! slot in a transaction, exporting the transaction id, and broadcasting the transaction
//! information to all other workers via a broadcasted feedback edge.
//!
//! During this phase the follower workers are simply waiting to hear on the feedback edge,
//! effectively synchronizing with the leader. Once all workers have received the snapshot
//! information they can all start to perform their assigned COPY queries.
//!
//! The leader and follower steps described above are accomplished by the [`export_snapshot`] and
//! [`use_snapshot`] functions respectively.
//!
//! ## Coordinated transaction COMMIT
//!
//! When follower workers are done with snapshotting they commit their transaction, close their
//! session, and then drop their snapshot feedback capability. When the leader worker is done with
//! snapshotting it drops its snapshot feedback capability and waits until it observes the
//! snapshot input advancing to the empty frontier. This allows the leader to COMMIT its
//! transaction last, which is the transaction that exported the snapshot.
//!
//! It's unclear if this is strictly necessary, but having the frontiers made it easy enough that I
//! added the synchronization.
//!
//! ## Snapshot rewinding
//!
//! Ingestion dataflows must produce definite data, including the snapshot. What this means
//! practically is that whenever we deem it necessary to snapshot a table we must do so at the same
//! LSN. However, the method for running a transaction described above doesn't let us choose the
//! LSN, it could be an LSN in the future chosen by PostgresSQL while it creates the temporary
//! replication slot.
//!
//! The definition of differential collections states that a collection at some time `t_snapshot`
//! is defined to be the accumulation of all updates that happen at `t <= t_snapshot`, where `<=`
//! is the partial order. In this case we are faced with the problem of knowing the state of a
//! table at `t_snapshot` but actually wanting to know the snapshot at `t_slot <= t_snapshot`.
//!
//! From the definition we can see that the snapshot at `t_slot` is related to the snapshot at
//! `t_snapshot` with the following equations:
//!
//!```text
//! sum(update: t <= t_snapshot) = sum(update: t <= t_slot) + sum(update: t_slot <= t <= t_snapshot)
//!                                         |
//!                                         V
//! sum(update: t <= t_slot) = sum(update: t <= snapshot) - sum(update: t_slot <= t <= t_snapshot)
//! ```
//!
//! Therefore, if we manage to recover the `sum(update: t_slot <= t <= t_snapshot)` term we will be
//! able to "rewind" the snapshot we obtained at `t_snapshot` to `t_slot` by emitting all updates
//! that happen between these two points with their diffs negated.
//!
//! It turns out that this term is exactly what the main replication slot provides us with and we
//! can rewind snapshot at arbitrary points! In order to do this the snapshot dataflow emits rewind
//! requests to the replication reader which informs it that a certain range of updates must be
//! emitted at LSN 0 (by convention) with their diffs negated. These negated diffs are consolidated
//! with the diffs taken at `t_snapshot` that were also emitted at LSN 0 (by convention) and we end
//! up with a TVC that at LSN 0 contains the snapshot at `t_slot`.
//!
//! # Snapshot decoding
//!
//! The expectation is that tables will most likely be skewed on the number of rows they contain so
//! while a `COPY` query for any given table runs on a single worker the decoding of the COPY
//! stream is distributed to all workers.
//!
//!
//! ```text
//!                 ╭──────────────────╮
//!    ┏━━━━━━━━━━━━v━┓                │ exported
//!    ┃    table     ┃   ╭─────────╮  │ snapshot id
//!    ┃    reader    ┠─>─┤broadcast├──╯
//!    ┗━┯━━━━━━━━━━┯━┛   ╰─────────╯
//!   raw│          │
//!  COPY│          │
//!  data│          │
//! ╭────┴─────╮    │
//! │distribute│    │
//! ╰────┬─────╯    │
//! ┏━━━━┷━━━━┓     │
//! ┃  COPY   ┃     │
//! ┃ decoder ┃     │
//! ┗━━━━┯━━━━┛     │
//!      │ snapshot │rewind
//!      │ updates  │requests
//!      v          v
//! ```

use std::collections::BTreeMap;
use std::convert::Infallible;
use std::pin::pin;
use std::rc::Rc;
use std::sync::Arc;
use std::time::Duration;

use anyhow::bail;
use differential_dataflow::AsCollection;
use futures::{StreamExt as _, TryStreamExt};
use mz_ore::cast::CastFrom;
use mz_ore::future::InTask;
use mz_postgres_util::tunnel::PostgresFlavor;
use mz_postgres_util::{Client, PostgresError, simple_query_opt};
use mz_repr::{Datum, DatumVec, Diff, Row};
use mz_sql_parser::ast::{Ident, display::AstDisplay};
use mz_storage_types::errors::DataflowError;
use mz_storage_types::parameters::PgSourceSnapshotConfig;
use mz_storage_types::sources::{MzOffset, PostgresSourceConnection};
use mz_timely_util::builder_async::{
    Event as AsyncEvent, OperatorBuilder as AsyncOperatorBuilder, PressOnDropButton,
};
use timely::container::CapacityContainerBuilder;
use timely::dataflow::channels::pact::{Exchange, Pipeline};
use timely::dataflow::operators::core::Map;
use timely::dataflow::operators::{
    Broadcast, CapabilitySet, Concat, ConnectLoop, Feedback, Operator,
};
use timely::dataflow::{Scope, Stream};
use timely::progress::Timestamp;
use tokio_postgres::error::SqlState;
use tokio_postgres::types::{Oid, PgLsn};
use tracing::trace;

use crate::metrics::source::postgres::PgSnapshotMetrics;
use crate::source::RawSourceCreationConfig;
use crate::source::postgres::replication::RewindRequest;
use crate::source::postgres::{
    DefiniteError, ReplicationError, SourceOutputInfo, TransientError, verify_schema,
};
use crate::source::types::{
    ProgressStatisticsUpdate, SignaledFuture, SourceMessage, StackedCollection,
};

/// Renders the snapshot dataflow. See the module documentation for more information.
pub(crate) fn render<G: Scope<Timestamp = MzOffset>>(
    mut scope: G,
    config: RawSourceCreationConfig,
    connection: PostgresSourceConnection,
    table_info: BTreeMap<u32, BTreeMap<usize, SourceOutputInfo>>,
    metrics: PgSnapshotMetrics,
) -> (
    StackedCollection<G, (usize, Result<SourceMessage, DataflowError>)>,
    Stream<G, RewindRequest>,
    Stream<G, Infallible>,
    Stream<G, ProgressStatisticsUpdate>,
    Stream<G, ReplicationError>,
    PressOnDropButton,
) {
    let op_name = format!("TableReader({})", config.id);
    let mut builder = AsyncOperatorBuilder::new(op_name, scope.clone());

    let (feedback_handle, feedback_data) = scope.feedback(Default::default());

    let (raw_handle, raw_data) = builder.new_output();
    let (rewinds_handle, rewinds) = builder.new_output();
    // This output is used to signal to the replication operator that the replication slot has been
    // created. With the current state of execution serialization there isn't a lot of benefit
    // of splitting the snapshot and replication phases into two operators.
    // TODO(petrosagg): merge the two operators in one (while still maintaining separation as
    // functions/modules)
    let (_, slot_ready) = builder.new_output::<CapacityContainerBuilder<_>>();
    let (snapshot_handle, snapshot) = builder.new_output();
    let (definite_error_handle, definite_errors) = builder.new_output();

    let (stats_output, stats_stream) = builder.new_output();

    // This operator needs to broadcast data to itself in order to synchronize the transaction
    // snapshot. However, none of the feedback capabilities result in output messages and for the
    // feedback edge specifically having a default conncetion would result in a loop.
    let mut snapshot_input = builder.new_disconnected_input(&feedback_data, Pipeline);

    // The export id must be sent to all workers, so we broadcast the feedback connection
    snapshot.broadcast().connect_loop(feedback_handle);

    let is_snapshot_leader = config.responsible_for("snapshot_leader");

    // A global view of all outputs that will be snapshot by all workers.
    let mut all_outputs = vec![];
    // A filtered table info containing only the tables that this worker should snapshot.
    let mut reader_table_info = BTreeMap::new();
    for (table, outputs) in table_info.iter() {
        for (&output_index, output) in outputs {
            if *output.resume_upper != [MzOffset::minimum()] {
                // Already has been snapshotted.
                continue;
            }
            all_outputs.push(output_index);
            if config.responsible_for(*table) {
                reader_table_info
                    .entry(*table)
                    .or_insert_with(BTreeMap::new)
                    .insert(output_index, (output.desc.clone(), output.casts.clone()));
            }
        }
    }

    let (button, transient_errors) = builder.build_fallible(move |caps| {
        let busy_signal = Arc::clone(&config.busy_signal);
        Box::pin(SignaledFuture::new(busy_signal, async move {
            let id = config.id;
            let worker_id = config.worker_id;

            let [
                data_cap_set,
                rewind_cap_set,
                slot_ready_cap_set,
                snapshot_cap_set,
                definite_error_cap_set,
                stats_cap,
            ]: &mut [_; 6] = caps.try_into().unwrap();

            trace!(
                %id,
                "timely-{worker_id} initializing table reader \
                    with {} tables to snapshot",
                    reader_table_info.len()
            );

            // Nothing needs to be snapshot.
            if all_outputs.is_empty() {
                trace!(%id, "no exports to snapshot");
                // Note we do not emit a `ProgressStatisticsUpdate::Snapshot` update here,
                // as we do not want to attempt to override the current value with 0. We
                // just leave it null.
                return Ok(());
            }

            let connection_config = connection
                .connection
                .config(
                    &config.config.connection_context.secrets_reader,
                    &config.config,
                    InTask::Yes,
                )
                .await?;
            let task_name = format!("timely-{worker_id} PG snapshotter");

            let client = if is_snapshot_leader {
                let client = connection_config
                    .connect_replication(&config.config.connection_context.ssh_tunnel_manager)
                    .await?;

                // Attempt to export the snapshot by creating the main replication slot. If that
                // succeeds then there is no need for creating additional temporary slots.
                let main_slot = &connection.publication_details.slot;
                let snapshot_info = match export_snapshot(&client, main_slot, false).await {
                    Ok(info) => info,
                    Err(err @ TransientError::ReplicationSlotAlreadyExists) => {
                        match connection.connection.flavor {
                            // If we're connecting to a vanilla we have the option of exporting a
                            // snapshot via a temporary slot
                            PostgresFlavor::Vanilla => {
                                let tmp_slot = format!(
                                    "mzsnapshot_{}",
                                    uuid::Uuid::new_v4()).replace('-', ""
                                );
                                export_snapshot(&client, &tmp_slot, true).await?
                            }
                            // No salvation for Yugabyte
                            PostgresFlavor::Yugabyte => return Err(err),
                        }
                    }
                    Err(err) => return Err(err),
                };
                trace!(
                    %id,
                    "timely-{worker_id} exporting snapshot info {snapshot_info:?}");
                snapshot_handle.give(&snapshot_cap_set[0], snapshot_info);

                client
            } else {
                // Only the snapshot leader needs a replication connection.
                connection_config
                    .connect(
                        &task_name,
                        &config.config.connection_context.ssh_tunnel_manager,
                    )
                    .await?
            };
            *slot_ready_cap_set = CapabilitySet::new();

            // Configure statement_timeout based on param. We want to be able to
            // override the server value here in case it's set too low,
            // respective to the size of the data we need to copy.
            set_statement_timeout(
                &client,
                config
                    .config
                    .parameters
                    .pg_source_snapshot_statement_timeout,
            )
            .await?;

            let (snapshot, snapshot_lsn) = loop {
                match snapshot_input.next().await {
                    Some(AsyncEvent::Data(_, mut data)) => {
                        break data.pop().expect("snapshot sent above")
                    }
                    Some(AsyncEvent::Progress(_)) => continue,
                    None => panic!(
                        "feedback closed \
                    before sending snapshot info"
                    ),
                }
            };
            // Snapshot leader is already in identified transaction but all other workers need to enter it.
            if !is_snapshot_leader {
                trace!(%id, "timely-{worker_id} using snapshot id {snapshot:?}");
                use_snapshot(&client, &snapshot).await?;
            }

            let upstream_info = {
                let schema_client = connection_config
                    .connect(
                        "snapshot schema info",
                        &config.config.connection_context.ssh_tunnel_manager,
                    )
                    .await?;
                match mz_postgres_util::publication_info(&schema_client, &connection.publication, Some(&reader_table_info.keys().copied().collect::<Vec<_>>()))
                    .await
                {
                    // If the replication stream cannot be obtained in a definite way there is
                    // nothing else to do. These errors are not retractable.
                    Err(PostgresError::PublicationMissing(publication)) => {
                        let err = DefiniteError::PublicationDropped(publication);
                        for (oid, outputs) in reader_table_info.iter() {
                            // Produce a definite error here and then exit to ensure
                            // a missing publication doesn't generate a transient
                            // error and restart this dataflow indefinitely.
                            //
                            // We pick `u64::MAX` as the LSN which will (in
                            // practice) never conflict any previously revealed
                            // portions of the TVC.
                            for output_index in outputs.keys() {
                                let update = (
                                    (*oid, *output_index, Err(err.clone().into())),
                                    MzOffset::from(u64::MAX),
                                    Diff::ONE,
                                );
                                raw_handle.give_fueled(&data_cap_set[0], update).await;
                            }
                        }

                        definite_error_handle.give(
                            &definite_error_cap_set[0],
                            ReplicationError::Definite(Rc::new(err)),
                        );
                        return Ok(());
                    }
                    Err(e) => Err(TransientError::from(e))?,
                    Ok(i) => i,
                }
            };

            let worker_tables = reader_table_info
                .iter()
                .map(|(_, outputs)| {
                    // just use the first output's desc since the fields accessed here should
                    // be the same for all outputs
                    let desc = &outputs.values().next().expect("at least 1").0;
                    (
                        format!(
                            "{}.{}",
                            Ident::new_unchecked(desc.namespace.clone()).to_ast_string_simple(),
                            Ident::new_unchecked(desc.name.clone()).to_ast_string_simple()
                        ),
                        desc.oid.clone(),
                        outputs.len(),
                    )
                })
                .collect();

            let snapshot_total =
                fetch_snapshot_size(&client, worker_tables, metrics, &config).await?;

            stats_output.give(
                &stats_cap[0],
                ProgressStatisticsUpdate::Snapshot {
                    records_known: snapshot_total,
                    records_staged: 0,
                },
            );

            let mut snapshot_staged = 0;
            for (&oid, outputs) in reader_table_info.iter() {
                let mut table_name = None;
                let mut output_indexes = vec![];
                for (output_index, (expected_desc, casts)) in outputs.iter() {
                    match verify_schema(oid, expected_desc, &upstream_info, casts) {
                        Ok(()) => {
                            if table_name.is_none() {
                                table_name = Some((
                                    expected_desc.namespace.clone(),
                                    expected_desc.name.clone(),
                                ));
                            }
                            output_indexes.push(output_index);
                        }
                        Err(err) => {
                            raw_handle
                                .give_fueled(
                                    &data_cap_set[0],
                                    (
                                        (oid, *output_index, Err(err.into())),
                                        MzOffset::minimum(),
                                       Diff::ONE,
                                    ),
                                )
                                .await;
                            continue;
                        }
                    };
                }

                let (namespace, table) = match table_name {
                    Some(t) => t,
                    None => {
                        // all outputs errored for this table
                        continue;
                    }
                };

                trace!(
                    %id,
                    "timely-{worker_id} snapshotting table {:?}({oid}) @ {snapshot_lsn}",
                    table
                );

                // To handle quoted/keyword names, we can use `Ident`'s AST printing, which
                // emulate's PG's rules for name formatting.
                let query = format!(
                    "COPY {}.{} TO STDOUT (FORMAT TEXT, DELIMITER '\t')",
                    Ident::new_unchecked(namespace).to_ast_string_simple(),
                    Ident::new_unchecked(table).to_ast_string_simple(),
                );
                let mut stream = pin!(client.copy_out_simple(&query).await?);

                let mut update = ((oid, 0, Ok(vec![])), MzOffset::minimum(), Diff::ONE);
                while let Some(bytes) = stream.try_next().await? {
                    let data = update.0 .2.as_mut().unwrap();
                    data.clear();
                    data.extend_from_slice(&bytes);
                    for output_index in &output_indexes {
                        update.0 .1 = **output_index;
                        raw_handle.give_fueled(&data_cap_set[0], &update).await;
                        snapshot_staged += 1;
                        // TODO(guswynn): does this 1000 need to be configurable?
                        if snapshot_staged % 1000 == 0 {
                            stats_output.give(
                                &stats_cap[0],
                                ProgressStatisticsUpdate::Snapshot {
                                    records_known: snapshot_total,
                                    records_staged: snapshot_staged,
                                },
                            );
                        }
                    }
                }
            }

            // We are done with the snapshot so now we will emit rewind requests. It is important
            // that this happens after the snapshot has finished because this is what unblocks the
            // replication operator and we want this to happen serially. It might seem like a good
            // idea to read the replication stream concurrently with the snapshot but it actually
            // leads to a lot of data being staged for the future, which needlesly consumed memory
            // in the cluster.
            for output in reader_table_info.values() {
                for (output_index, (desc, _)) in output {
                    trace!(%id, "timely-{worker_id} producing rewind request for table {} output {output_index}", desc.name);
                    let req = RewindRequest { output_index: *output_index, snapshot_lsn };
                    rewinds_handle.give(&rewind_cap_set[0], req);
                }
            }
            *rewind_cap_set = CapabilitySet::new();

            // Report the same known and staged records to signify that the snapshot is complete.
            stats_output.give(
                &stats_cap[0],
                ProgressStatisticsUpdate::Snapshot {
                    records_known: snapshot_staged,
                    records_staged: snapshot_staged,
                },
            );

            // Failure scenario after we have produced the snapshot, but before a successful COMMIT
            fail::fail_point!("pg_snapshot_failure", |_| Err(
                TransientError::SyntheticError
            ));

            // The exporting worker should wait for all the other workers to commit before dropping
            // its client since this is what holds the exported transaction alive.
            if is_snapshot_leader {
                trace!(%id, "timely-{worker_id} waiting for all workers to finish");
                *snapshot_cap_set = CapabilitySet::new();
                while snapshot_input.next().await.is_some() {}
                trace!(%id, "timely-{worker_id} (leader) comitting COPY transaction");
                client.simple_query("COMMIT").await?;
            } else {
                trace!(%id, "timely-{worker_id} comitting COPY transaction");
                client.simple_query("COMMIT").await?;
                *snapshot_cap_set = CapabilitySet::new();
            }
            drop(client);
            Ok(())
        }))
    });

    // We now decode the COPY protocol and apply the cast expressions
    let mut text_row = Row::default();
    let mut final_row = Row::default();
    let mut datum_vec = DatumVec::new();
    let mut next_worker = (0..u64::cast_from(scope.peers()))
        // Round robin on 1000-records basis to avoid creating tiny containers when there are a
        // small number of updates and a large number of workers.
        .flat_map(|w| std::iter::repeat_n(w, 1000))
        .cycle();
    let round_robin = Exchange::new(move |_| next_worker.next().unwrap());
    let snapshot_updates = raw_data
        .map::<Vec<_>, _, _>(Clone::clone)
        .unary(round_robin, "PgCastSnapshotRows", |_, _| {
            move |input, output| {
                while let Some((time, data)) = input.next() {
                    let mut session = output.session(&time);
                    for ((oid, output_index, event), time, diff) in data.drain(..) {
                        let output = &table_info
                            .get(&oid)
                            .and_then(|outputs| outputs.get(&output_index))
                            .expect("table_info contains all outputs");

                        let event = event
                            .as_ref()
                            .map_err(|e: &DataflowError| e.clone())
                            .and_then(|bytes| {
                                decode_copy_row(bytes, output.casts.len(), &mut text_row)?;
                                let datums = datum_vec.borrow_with(&text_row);
                                super::cast_row(&output.casts, &datums, &mut final_row)?;
                                Ok(SourceMessage {
                                    key: Row::default(),
                                    value: final_row.clone(),
                                    metadata: Row::default(),
                                })
                            });

                        session.give(((output_index, event), time, diff));
                    }
                }
            }
        })
        .as_collection();

    let errors = definite_errors.concat(&transient_errors.map(ReplicationError::from));

    (
        snapshot_updates,
        rewinds,
        slot_ready,
        stats_stream,
        errors,
        button.press_on_drop(),
    )
}

/// Starts a read-only transaction on the SQL session of `client` at a consistent LSN point by
/// creating a replication slot. Returns a snapshot identifier that can be imported in
/// other SQL session and the LSN of the consistent point.
async fn export_snapshot(
    client: &Client,
    slot: &str,
    temporary: bool,
) -> Result<(String, MzOffset), TransientError> {
    match export_snapshot_inner(client, slot, temporary).await {
        Ok(ok) => Ok(ok),
        Err(err) => {
            // We don't want to leave the client inside a failed tx
            client.simple_query("ROLLBACK;").await?;
            Err(err)
        }
    }
}

async fn export_snapshot_inner(
    client: &Client,
    slot: &str,
    temporary: bool,
) -> Result<(String, MzOffset), TransientError> {
    client
        .simple_query("BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ;")
        .await?;

    // Note: Using unchecked here is okay because we're using it in a SQL query.
    let slot = Ident::new_unchecked(slot).to_ast_string_simple();
    let temporary_str = if temporary { " TEMPORARY" } else { "" };
    let query =
        format!("CREATE_REPLICATION_SLOT {slot}{temporary_str} LOGICAL \"pgoutput\" USE_SNAPSHOT");
    let row = match simple_query_opt(client, &query).await {
        Ok(row) => Ok(row.unwrap()),
        Err(PostgresError::Postgres(err)) if err.code() == Some(&SqlState::DUPLICATE_OBJECT) => {
            return Err(TransientError::ReplicationSlotAlreadyExists);
        }
        Err(err) => Err(err),
    }?;

    // When creating a replication slot postgres returns the LSN of its consistent point, which is
    // the LSN that must be passed to `START_REPLICATION` to cleanly transition from the snapshot
    // phase to the replication phase. `START_REPLICATION` includes all transactions that commit at
    // LSNs *greater than or equal* to the passed LSN. Therefore the snapshot phase must happen at
    // the greatest LSN that is not beyond the consistent point. That LSN is `consistent_point - 1`
    let consistent_point: PgLsn = row.get("consistent_point").unwrap().parse().unwrap();
    let consistent_point = u64::from(consistent_point)
        .checked_sub(1)
        .expect("consistent point is always non-zero");

    let row = simple_query_opt(client, "SELECT pg_export_snapshot();")
        .await?
        .unwrap();
    let snapshot = row.get("pg_export_snapshot").unwrap().to_owned();

    Ok((snapshot, MzOffset::from(consistent_point)))
}

/// Starts a read-only transaction on the SQL session of `client` at a the consistent LSN point of
/// `snapshot`.
async fn use_snapshot(client: &Client, snapshot: &str) -> Result<(), TransientError> {
    client
        .simple_query("BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ;")
        .await?;
    let query = format!("SET TRANSACTION SNAPSHOT '{snapshot}';");
    client.simple_query(&query).await?;
    Ok(())
}

async fn set_statement_timeout(client: &Client, timeout: Duration) -> Result<(), TransientError> {
    // Value is known to accept milliseconds w/o units.
    // https://www.postgresql.org/docs/current/runtime-config-client.html
    client
        .simple_query(&format!("SET statement_timeout = {}", timeout.as_millis()))
        .await?;
    Ok(())
}

/// Decodes a row of `col_len` columns obtained from a text encoded COPY query into `row`.
fn decode_copy_row(data: &[u8], col_len: usize, row: &mut Row) -> Result<(), DefiniteError> {
    let mut packer = row.packer();
    let row_parser = mz_pgcopy::CopyTextFormatParser::new(data, b'\t', "\\N");
    let mut column_iter = row_parser.iter_raw_truncating(col_len);
    for _ in 0..col_len {
        let value = match column_iter.next() {
            Some(Ok(value)) => value,
            Some(Err(_)) => return Err(DefiniteError::InvalidCopyInput),
            None => return Err(DefiniteError::MissingColumn),
        };
        let datum = value.map(super::decode_utf8_text).transpose()?;
        packer.push(datum.unwrap_or(Datum::Null));
    }
    Ok(())
}

/// Record the sizes of the tables being snapshotted in `PgSnapshotMetrics`.
async fn fetch_snapshot_size(
    client: &Client,
    // The table names, oids, and number of outputs for this table owned by this worker.
    tables: Vec<(String, Oid, usize)>,
    metrics: PgSnapshotMetrics,
    config: &RawSourceCreationConfig,
) -> Result<u64, anyhow::Error> {
    // TODO(guswynn): delete unused configs
    let snapshot_config = config.config.parameters.pg_snapshot_config;

    let mut total = 0;
    for (table, oid, output_count) in tables {
        let stats = collect_table_statistics(client, snapshot_config, &table, oid).await?;
        metrics.record_table_count_latency(table, stats.count_latency);
        total += stats.count * u64::cast_from(output_count);
    }
    Ok(total)
}

#[derive(Default)]
struct TableStatistics {
    count: u64,
    count_latency: f64,
}

async fn collect_table_statistics(
    client: &Client,
    config: PgSourceSnapshotConfig,
    table: &str,
    oid: u32,
) -> Result<TableStatistics, anyhow::Error> {
    use mz_ore::metrics::MetricsFutureExt;
    let mut stats = TableStatistics::default();

    let estimate_row = simple_query_opt(
        client,
        &format!("SELECT reltuples::bigint AS estimate_count FROM pg_class WHERE oid = '{oid}'"),
    )
    .wall_time()
    .set_at(&mut stats.count_latency)
    .await?;
    stats.count = match estimate_row {
        Some(row) => row.get("estimate_count").unwrap().parse().unwrap_or(0),
        None => bail!("failed to get estimate count for {table}"),
    };

    // If the estimate is low enough we can attempt to get an exact count. Note that not yet
    // vacuumed tables will report zero rows here and there is a possibility that they are very
    // large. We accept this risk and we offer the feature flag as an escape hatch if it becomes
    // problematic.
    if config.collect_strict_count && stats.count < 1_000_000 {
        let count_row = simple_query_opt(client, &format!("SELECT count(*) as count from {table}"))
            .wall_time()
            .set_at(&mut stats.count_latency)
            .await?;
        stats.count = match count_row {
            Some(row) => row.get("count").unwrap().parse().unwrap(),
            None => bail!("failed to get count for {table}"),
        }
    }

    Ok(stats)
}