Proposal - Allow extensions to set a Plan Identifier

Hi,

A patch by Lukas Fittl [1] introduces the pg_stat_plans extension, which
exposes execution plans along with execution statistics. As part of this work,
a new infrastructure is being proposed to compute a plan identifier (planId),
which is exposed in both pg_stat_plans and pg_stat_activity.

Exposing planId in pg_stat_activity enables users to identify long-running
or high-load plans and correlate them with plan text from the extension.

One of open question in [1] is which components of a plan tree should be
considered when computing planId. Lukas has created a wiki [2] for discussion,
but reaching a consensus may take time—possibly in time for v19, but
it's uncertain.

Meanwhile, existing extensions like pg_stat_monitor [3] compute a planId and
store the plan text, but they lack a way to expose planId in pg_stat_activity.
This limits their usefulness, as identifying top or long-running plans from
pg_stat_activity is critical for monitoring.

To address this, I propose leveraging work from [1] and allow extensions
to set a planId This could be implemented sooner than an in-core
computed planId.

Proposal Overview:

1/ Add a planId field to PgBackendStatus.
2/ Add a planId field to PlannedStmt.
3/ Create APIs for extensions to set the current planId.

Storing planId in PlannedStmt ensures it is available with
cached plans.

One consideration is whether reserving a 64-bit integer in PgBackendStatus
and PlannedStmt is acceptable, given that planId may not always be used.
However, this is already the case for queryId when compute_query_id is
disabled and no extension sets it, so it may not be a concern.

Looking forward to feedback on this approach.
If there is agreement, I will work on preparing the patches.

Regards,

Sami Imseih
Amazon Web Services (AWS)

[1] https://www.postgresql.org/message-id/flat/CAP53Pkyow59ajFMHGpmb1BK9WHDypaWtUsS_5DoYUEfsa_Hktg%40mail.gmail.com
[2] https://www.postgresql.org/message-id/CAP53PkxocbNr%2BeRag3FEJp3-7S1U80FspOg8UQjO902TWMG%3D6A%40mail.gmail.com
[3] https://github.com/percona/pg_stat_monitor