Re: Yet another fast GiST build

From: Andrey Borodin <x4mmm(at)yandex-team(dot)ru>
To: Heikki Linnakangas <hlinnaka(at)iki(dot)fi>
Cc: Peter Geoghegan <pg(at)bowt(dot)ie>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Justin Pryzby <pryzby(at)telsasoft(dot)com>, Darafei Komяpa Praliaskouski <me(at)komzpa(dot)net>, Pavel Borisov <pashkin(dot)elfe(at)gmail(dot)com>, Thomas Munro <thomas(dot)munro(at)gmail(dot)com>, Michael Paquier <michael(at)paquier(dot)xyz>, Alexander Korotkov <a(dot)korotkov(at)postgrespro(dot)ru>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>, Ibrar Ahmed <ibrar(dot)ahmad(at)gmail(dot)com>
Subject: Re: Yet another fast GiST build
Date: 2021-04-07 12:12:21
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> 7 апр. 2021 г., в 13:23, Heikki Linnakangas <hlinnaka(at)iki(dot)fi> написал(а):
> Committed with small fixes.


> 7 апр. 2021 г., в 14:56, Heikki Linnakangas <hlinnaka(at)iki(dot)fi> написал(а):
> Ok, I think I understand that now. In btree_gist, the *_cmp() function operates on non-leaf values, and *_lt(), *_gt() et al operate on leaf values. For all other datatypes, the leaf and non-leaf representation is the same, but for bit/varbit, the non-leaf representation is different. The leaf representation is VarBit, and non-leaf is just the bits without the 'bit_len' field. That's why it is indeed correct for gbt_bitcmp() to just use byteacmp(), whereas gbt_bitlt() et al compares the 'bit_len' field separately. That's subtle, and 100% uncommented.
> What that means for this patch is that gbt_bit_sort_build_cmp() should *not* call byteacmp(), but bitcmp(). Because it operates on the original datatype stored in the table.

Thanks for investigating this.
If I understand things right, adding test values with different lengths of bit sequences would not uncover the problem anyway?

Best regards, Andrey Borodin.

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