From: Michael Paquier <michael@paquier.xyz>
To: pgsql-committers@lists.postgresql.org
Subject: pgsql: Rework output format of pg_ndistinct
Date: Mon, 17 Nov 2025 00:52:43 +0000
Message-ID: <E1vKnU3-006y4F-0K@gemulon.postgresql.org> (raw)
Rework output format of pg_ndistinct
The existing format of pg_ndistinct uses a single-object JSON structure
where each key is itself a comma-separated list of attnums, like:
{"3, 4": 11, "3, 6": 11, "4, 6": 11, "3, 4, 6": 11}
While this is a very compact format, it is confusing to read and it is
difficult to manipulate the values within the object.
The new output format introduced in this commit is an array of objects,
with:
- A key named "attributes", that contains an array of attribute numbers.
- A key named "ndistinct", represented as an integer.
The values use the same underlying type as previously when printed, with
a new output format that shows now as follows:
[{"ndistinct": 11, "attributes": [3,4]},
{"ndistinct": 11, "attributes": [3,6]},
{"ndistinct": 11, "attributes": [4,6]},
{"ndistinct": 11, "attributes": [3,4,6]}]
This new format will become handy for a follow-up set of changes, so as
it becomes possible to inject extended statistics rather than require an
ANALYZE, like in a dump/restore sequence or after pg_upgrade on a new
cluster.
This format has been suggested by Tomas Vondra. The key names are
defined in a new header, to ease with the integration of
frontend-specific changes that are still under discussion. (Personal
note: I am not specifically wedded to these key names, but if there are
better name suggestions for this release, feel free.)
The bulk of the changes come from the regression tests, where
jsonb_pretty() is now used to make the outputs generated easier to
parse.
Author: Corey Huinker <corey.huinker@gmail.com>
Reviewed-by: Jian He <jian.universality@gmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/CADkLM=dpz3KFnqP-dgJ-zvRvtjsa8UZv8wDAQdqho=qN3kX0Zg@mail.gmail.com
Branch
------
master
Details
-------
https://git.postgresql.org/pg/commitdiff/1f927cce44983ed59a3c1eccc95ad2946ac13b42
Modified Files
--------------
doc/src/sgml/perform.sgml | 38 ++++++-
src/backend/utils/adt/pg_ndistinct.c | 22 ++--
src/include/statistics/statistics_format.h | 32 ++++++
src/test/regress/expected/stats_ext.out | 156 +++++++++++++++++++++++++----
src/test/regress/sql/stats_ext.sql | 12 +--
5 files changed, 223 insertions(+), 37 deletions(-)
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To: pgsql-committers@postgresql.org
Cc: michael@paquier.xyz, pgsql-committers@lists.postgresql.org
Subject: Re: pgsql: Rework output format of pg_ndistinct
In-Reply-To: <E1vKnU3-006y4F-0K@gemulon.postgresql.org>
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