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Remove the "snapshot too old" feature.
Remove the old_snapshot_threshold setting and mechanism for producing the error "snapshot too old", originally added by commit 848ef42. Unfortunately it had a number of known problems in terms of correctness and performance, mostly reported by Andres in the course of his work on snapshot scalability. We agreed to remove it, after a long period without an active plan to fix it. This is certainly a desirable feature, and someone might propose a new or improved implementation in the future. Reported-by: Andres Freund <andres@anarazel.de> Discussion: https://postgr.es/m/CACG%3DezYV%2BEvO135fLRdVn-ZusfVsTY6cH1OZqWtezuEYH6ciQA%40mail.gmail.com Discussion: https://postgr.es/m/20200401064008.qob7bfnnbu4w5cw4%40alap3.anarazel.de Discussion: https://postgr.es/m/CA%2BTgmoY%3Daqf0zjTD%2B3dUWYkgMiNDegDLFjo%2B6ze%3DWtpik%2B3XqA%40mail.gmail.com
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‎contrib/Makefile

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@@ -29,7 +29,6 @@ SUBDIRS = \
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lo \
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ltree \
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oid2name \
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old_snapshot \
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pageinspect \
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passwordcheck \
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pg_buffercache \

‎contrib/bloom/blscan.c

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@@ -132,7 +132,6 @@ blgetbitmap(IndexScanDesc scan, TIDBitmap *tbm)
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LockBuffer(buffer, BUFFER_LOCK_SHARE);
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page = BufferGetPage(buffer);
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TestForOldSnapshot(scan->xs_snapshot, scan->indexRelation, page);
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if (!PageIsNew(page) && !BloomPageIsDeleted(page))
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{

‎contrib/meson.build

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@@ -37,7 +37,6 @@ subdir('lo')
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subdir('ltree')
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subdir('ltree_plpython')
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subdir('oid2name')
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subdir('old_snapshot')
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subdir('pageinspect')
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subdir('passwordcheck')
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subdir('pg_buffercache')

‎contrib/old_snapshot/Makefile

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‎contrib/old_snapshot/meson.build

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‎contrib/old_snapshot/old_snapshot--1.0.sql

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‎contrib/old_snapshot/old_snapshot.control

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‎contrib/old_snapshot/time_mapping.c

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‎doc/src/sgml/config.sgml

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@@ -2709,65 +2709,6 @@ include_dir 'conf.d'
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</para>
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</listitem>
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</varlistentry>
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<varlistentry id="guc-old-snapshot-threshold" xreflabel="old_snapshot_threshold">
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<term><varname>old_snapshot_threshold</varname> (<type>integer</type>)
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<indexterm>
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<primary><varname>old_snapshot_threshold</varname> configuration parameter</primary>
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</indexterm>
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</term>
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<listitem>
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<para>
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Sets the minimum amount of time that a query snapshot can be used
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without risk of a <quote>snapshot too old</quote> error occurring
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when using the snapshot. Data that has been dead for longer than
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this threshold is allowed to be vacuumed away. This can help
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prevent bloat in the face of snapshots which remain in use for a
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long time. To prevent incorrect results due to cleanup of data which
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would otherwise be visible to the snapshot, an error is generated
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when the snapshot is older than this threshold and the snapshot is
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used to read a page which has been modified since the snapshot was
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built.
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</para>
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<para>
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If this value is specified without units, it is taken as minutes.
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A value of <literal>-1</literal> (the default) disables this feature,
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effectively setting the snapshot age limit to infinity.
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This parameter can only be set at server start.
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</para>
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<para>
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Useful values for production work probably range from a small number
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of hours to a few days. Small values (such as <literal>0</literal> or
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<literal>1min</literal>) are only allowed because they may sometimes be
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useful for testing. While a setting as high as <literal>60d</literal> is
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allowed, please note that in many workloads extreme bloat or
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transaction ID wraparound may occur in much shorter time frames.
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</para>
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<para>
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When this feature is enabled, freed space at the end of a relation
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cannot be released to the operating system, since that could remove
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information needed to detect the <quote>snapshot too old</quote>
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condition. All space allocated to a relation remains associated with
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that relation for reuse only within that relation unless explicitly
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freed (for example, with <command>VACUUM FULL</command>).
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</para>
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<para>
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This setting does not attempt to guarantee that an error will be
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generated under any particular circumstances. In fact, if the
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correct results can be generated from (for example) a cursor which
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has materialized a result set, no error will be generated even if the
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underlying rows in the referenced table have been vacuumed away.
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Some tables cannot safely be vacuumed early, and so will not be
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affected by this setting, such as system catalogs. For such tables
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this setting will neither reduce bloat nor create a possibility
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of a <quote>snapshot too old</quote> error on scanning.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</sect2>
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</sect1>
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until it eventually reaches the primary. Standbys make no other use
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of feedback they receive other than to pass upstream.
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</para>
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<para>
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This setting does not override the behavior of
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<varname>old_snapshot_threshold</varname> on the primary; a snapshot on the
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standby which exceeds the primary's age threshold can become invalid,
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resulting in cancellation of transactions on the standby. This is
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because <varname>old_snapshot_threshold</varname> is intended to provide an
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absolute limit on the time which dead rows can contribute to bloat,
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which would otherwise be violated because of the configuration of a
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standby.
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</para>
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</listitem>
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</varlistentry>
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‎doc/src/sgml/contrib.sgml

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@@ -150,7 +150,6 @@ CREATE EXTENSION <replaceable>extension_name</replaceable>;
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&isn;
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&lo;
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&ltree;
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&oldsnapshot;
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&pageinspect;
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&passwordcheck;
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&pgbuffercache;

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