Re: Patch: Write Amplification Reduction Method (WARM)

From: Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
To: Pavan Deolasee <pavan(dot)deolasee(at)gmail(dot)com>
Cc: Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Bruce Momjian <bruce(at)momjian(dot)us>, Jaime Casanova <jaime(dot)casanova(at)2ndquadrant(dot)com>, Haribabu Kommi <kommi(dot)haribabu(at)gmail(dot)com>, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Patch: Write Amplification Reduction Method (WARM)
Date: 2017-03-29 06:22:31
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On Tue, Mar 28, 2017 at 10:35 PM, Pavan Deolasee
<pavan(dot)deolasee(at)gmail(dot)com> wrote:
> On Tue, Mar 28, 2017 at 7:04 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
> wrote:
>> For such an heap insert, we will pass
>> the actual value of column to index_form_tuple during index insert.
>> However during recheck when we fetch the value of c2 from heap tuple
>> and pass it index tuple, the value is already in compressed form and
>> index_form_tuple might again try to compress it because the size will
>> still be greater than TOAST_INDEX_TARGET and if it does so, it might
>> make recheck fail.
> Would it? I thought "if
> (!VARATT_IS_EXTENDED(DatumGetPointer(untoasted_values[i]))" check should
> prevent that. But I could be reading those macros wrong. They are probably
> heavily uncommented and it's not clear what each of those VARATT_* macro do.

That won't handle the case where it is simply compressed. You need
check like VARATT_IS_COMPRESSED to take care of compressed heap
tuples, but then also it won't work because heap_tuple_fetch_attr()
doesn't handle compressed tuples. You need to use
heap_tuple_untoast_attr() to handle the compressed case. Also, we
probably need to handle other type of var attrs. Now, If we want to
do all of that index_form_tuple() might not be the right place, we
probably want to handle it in caller or provide an alternate API.

With Regards,
Amit Kapila.

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