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Add show_data option to pg_get_wal_block_info.

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Allow users to opt out of returning FPI data and block data from
pg_get_wal_block_info as an optimization.  Testing has shown that this
can make function execution over twice as fast in some cases.

When pg_get_wal_block_info is called with "show_data := false", it
always returns NULL values for its block_data and block_fpi_data bytea
output parameters.  Nothing else changes.  In particular, the function
will still return the usual per-block summary of block data/FPI space
overhead.  Use of "show_data := false" is therefore feasible with all
queries that don't specifically require these raw binary strings.

Follow-up to recent work in commit 122376f0.  There still hasn't been a
stable release with the pg_get_wal_block_info function, so no bump in
the pg_walinspect extension version.

Per suggestion from Melanie Plageman.

Author: Peter Geoghegan <pg@bowt.ie>
Discussion: https://postgr.es/m/CAAKRu_bJvbcYBRj2cN6G2xV7B7-Ja+pjTO1nEnEhRR8OXYiABA@mail.gmail.com
Discussion: https://postgr.es/m/CAH2-Wzm9shOkEDM10_+qOZkRSQhKVxwBFiehH6EHWQQRd_rDPw@mail.gmail.com
This commit is contained in:
Peter Geoghegan
2023-03-31 14:02:52 -07:00
parent 6ee30209a6
commit df4f3ab517
6 changed files with 39 additions and 24 deletions
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17
doc/src/sgml/pgwalinspect.sgml
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@ -133,7 +133,7 @@ block_ref |
<varlistentry>
<term>
<function>pg_get_wal_block_info(start_lsn pg_lsn, end_lsn pg_lsn) returns setof record</function>
<function>pg_get_wal_block_info(start_lsn pg_lsn, end_lsn pg_lsn, show_data boolean DEFAULT true) returns setof record</function>
</term>
<listitem>
@ -209,10 +209,21 @@ block_fpi_data |
<para>
The <function>pg_filenode_relation</function> function (see
<xref linkend="functions-admin-dblocation"/>) can help you to
determine which block/relation was modified by each WAL record
during original execution
determine which relation was modified during original execution
</para>
</tip>
<para>
It is possible for clients to avoid the overhead of
materializing block data. This may make function execution
significantly faster. When <replaceable>show_data</replaceable>
is set to <literal>false</literal>, <structfield>block_data</structfield>
and <structfield>block_fpi_data</structfield> values are omitted
(that is, the <structfield>block_data</structfield> and
<structfield>block_fpi_data</structfield> <literal>OUT</literal>
arguments are <literal>NULL</literal> for all rows returned).
Obviously, this optimization is only feasible with queries where
block data isn't truly required.
</para>
<para>
The function raises an error if
<replaceable>start_lsn</replaceable> is not available.