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Date: Thu, 15 Aug 2024 15:48:19 +0200
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Subject: Re: Parallel CREATE INDEX for BRIN indexes
Cc: Andres Freund <andres@anarazel.de>,
 Matthias van de Meent <boekewurm+postgres@gmail.com>,
 PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
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From: Peter Eisentraut <peter@eisentraut.org>
To: Tomas Vondra <tomas@vondra.me>
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On 13.04.24 23:04, Tomas Vondra wrote:
>>> While preparing a differential code coverage report between 16 and HEAD, one
>>> thing that stands out is the parallel brin build code. Neither on
>>> coverage.postgresql.org nor locally is that code reached during our tests.
>>>
>>
>> Thanks for pointing this out, it's definitely something that I need to
>> improve (admittedly, should have been part of the patch). I'll also look
>> into eliminating the difference between BTREE and BRIN parallel builds,
>> mentioned in my last message in this thread.
>>
> 
> Here's a couple patches adding a test for the parallel CREATE INDEX with
> BRIN. The actual test is 0003/0004 - I added the test to pageinspect,
> because that allows cross-checking the index to one built without
> parallelism, which I think is better than just doing CREATE INDEX
> without properly testing it produces correct results.

These pageinspect tests added a new use of the md5() function.  We got
rid of those in the tests for PG17.  You could write the test case with
something like

  SELECT (CASE WHEN (mod(i,231) = 0) OR (i BETWEEN 3500 AND 4000) THEN NULL ELSE i END),
-       (CASE WHEN (mod(i,233) = 0) OR (i BETWEEN 3750 AND 4250) THEN NULL ELSE md5(i::text) END),
+       (CASE WHEN (mod(i,233) = 0) OR (i BETWEEN 3750 AND 4250) THEN NULL ELSE encode(sha256(i::text::bytea), 'hex') END),
         (CASE WHEN (mod(i,233) = 0) OR (i BETWEEN 3850 AND 4500) THEN NULL ELSE (i/100) + mod(i,8) END)

But this changes the test output slightly and I'm not sure if this gives
you the data distribution that you need for you test.  Could your check
this please?