Add immutable version of ash_raise_error function to support extensions like Citus
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This improves support for Citus (distributed Postgres) where it only supports immutable functions within CASE statements. A row-dependent token is passed in as an ignored argument to prevent the query planner from constant-folding the ash_raise_error_immutable call.
ash_sql will look for the :immutable_expr_error? key and, if true, use the new ash_raise_error_immutable function instead of ash_raise_error.
7 tasks
stevebrambilla
commented
Sep 25, 2025
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| def drop(2) do | ||
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So I like this, but I also hesitate to define this function automatically in everyone's databases when its only need by a few. My suggestion would be to create a new "extension" (not an Ash extension, just one of these postgres extension modules) that users can optionally add to define this function. |
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We use the Citus extension for sharding our Postgres database, which adds a requirement that any functions used within CASE or COALESCE expressions must be IMMUTABLE. Currently this means that we can't use error expressions at all because
ash_raise_erroris currently marked as STABLE.The current
ash_raise_errorcould technically be IMMUTABLE as it meets all of the requirements. However, the postgres planner will constant-fold the function call, making all expressions that use it immediately raise an exception. To prevent this we need to make the function call dependent on the row so it can't be constant-folded.This PR adds a new function,
ash_raise_error_immutable, that is marked IMMUTABLE and accepts a third argument that is simply ignored. This works together with a change in AshSql that adds a row-dependent argument, preventing the caching.It's opt-in only via an
AshPostgres.Repocallback, which stores a value in the repo config. AshSql looks for that config value when building the query and decides whether to call the existingash_raise_errorfunction, or the newash_raise_error_immutable.Related AshSql PR: ash-project/ash_sql#175
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