Module mz_sql::session::vars

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Run-time configuration parameters

§Overview

Materialize roughly follows the PostgreSQL configuration model, which works as follows. There is a global set of named configuration parameters, like DateStyle and client_encoding. These parameters can be set in several places: in an on-disk configuration file (in Postgres, named postgresql.conf), in command line arguments when the server is started, or at runtime via the ALTER SYSTEM or SET statements. Parameters that are set in a session take precedence over database defaults, which in turn take precedence over command line arguments, which in turn take precedence over settings in the on-disk configuration. Note that changing the value of parameters obeys transaction semantics: if a transaction fails to commit, any parameters that were changed in that transaction (i.e., via SET) will be rolled back to their previous value.

The Materialize configuration hierarchy at the moment is much simpler. Global defaults are hardcoded into the binary, and a select few parameters can be overridden per session. A select few parameters can be overridden on disk.

The set of variables that can be overridden per session and the set of variables that can be overridden on disk are currently disjoint. The infrastructure has been designed with an eye towards merging these two sets and supporting additional layers to the hierarchy, however, should the need arise.

The configuration parameters that exist are driven by compatibility with PostgreSQL drivers that expect them, not because they are particularly important.

§Structure

Thw most meaningful exports from this module are:

  • SessionVars represent per-session parameters, which each user can access independently of one another, and are accessed via SET.

    The fields of SessionVars are either;

    • SessionVar, which is preferable and simply requires full support of the SessionVar impl for its embedded value type.
    • ServerVar for types that do not currently support everything required by SessionVar, e.g. they are fixed-value parameters.

    In the fullness of time, all fields in SessionVars should be SessionVar.

  • SystemVars represent system-wide configuration settings and are accessed via ALTER SYSTEM SET.

    All elements of SystemVars are SystemVar.

Some VarDefinition are also marked as a FeatureFlag; this is just a wrapper to make working with a set of VarDefinition easier, primarily from within SQL planning, where we might want to check if a feature is enabled before planning it.

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Traits§

  • AsAny
    Helper trait …
  • Value
    Defines a value that get stored as part of a System or Session variable.
  • Var
    A Var represents a configuration parameter of an arbitrary type.

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