RE: Statistics Import and Export

Hi,
Thank you for developing this great feature. I have tested the committed feature.
The manual for the pg_set_relation_stats function says the following:
"The value of relpages must be greater than or equal to 0"

However, this function seems to accept -1 for the relpages parameter. Below is an example of execution:
---
postgres=> CREATE TABLE data1(c1 INT PRIMARY KEY, c2 VARCHAR(10));
CREATE TABLE
postgres=> SELECT pg_set_relation_stats('data1', relpages=>-1);
pg_set_relation_stats
-----------------------
t
(1 row)
postgres=> SELECT relname, relpages FROM pg_class WHERE relname='data1';
relname | relpages
---------+----------
data1 | -1
(1 row)
---

The attached patch modifies the pg_set_relation_stats function to work as described in the manual.

Regards,
Noriyoshi Shinoda

-----Original Message-----
From: Jeff Davis <pgsql(at)j-davis(dot)com>
Sent: Saturday, October 12, 2024 8:11 AM
To: jian he <jian(dot)universality(at)gmail(dot)com>; Corey Huinker <corey(dot)huinker(at)gmail(dot)com>
Cc: Matthias van de Meent <boekewurm+postgres(at)gmail(dot)com>; Bruce Momjian <bruce(at)momjian(dot)us>; Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>; Nathan Bossart <nathandbossart(at)gmail(dot)com>; Magnus Hagander <magnus(at)hagander(dot)net>; Stephen Frost <sfrost(at)snowman(dot)net>; Ashutosh Bapat <ashutosh(dot)bapat(dot)oss(at)gmail(dot)com>; Peter Smith <smithpb2250(at)gmail(dot)com>; PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>; Tomas Vondra <tomas(dot)vondra(at)enterprisedb(dot)com>; alvherre(at)alvh(dot)no-ip(dot)org
Subject: Re: Statistics Import and Export

On Mon, 2024-09-23 at 08:57 +0800, jian he wrote:
>     newtup = heap_modify_tuple_by_cols(ctup, tupdesc, ncols, replaces,
> nulls);
>
> you just directly declared "bool nulls[3]    = {false, false, false};"

Those must be false (not NULL), because in pg_class those are non-NULL attributes. They must be set to something whenever we update.

> if any of (RELPAGES_ARG, RELTUPLES_ARG, RELALLVISIBLE_ARG) is null,
> should you set that null[position] to true?

If the corresponding SQL argument is NULL, we leave the existing value unchanged, we don't set it to NULL.

> otherwise, i am confused with the variable nulls.
>
> Looking at other usage of heap_modify_tuple_by_cols, "ncols" cannot be
> dynamic, it should be a fixed value?
> The current implementation works, because the (bool[3] nulls) is
> always false, never changed.
> if nulls becomes {false, false, true} then "ncols" must be 3, cannot
> be 2.

heap_modify_tuple_by_cols() uses ncols to specify the length of the values/isnull arrays. The "replaces" is an array of attribute numbers to replace (in contrast to plain heap_modify_tuple(), which uses an array of booleans).

We are going to replace a maximum of 3 attributes, so the arrays have a maximum size of 3. Predeclaring the arrays to be 3 elements is just fine even if we only use the first 1-2 elements -- it avoids a needless heap allocation/free.

Regards,
Jeff Davis