postgres.git / summary / log / commit / refs

commit    5698f07947396e1a7cd6564390306aafa26fc189
Author:   Peter Geoghegan <pg@bowt.ie>
Date:     Mon May 08 20:17:31 2023 +0000
First released in: 16.0

    Doc: update VACUUM FREEZE query conflict guidance.
    
    Commit 1de58df4, which added page-level freezing, taught VACUUM to reuse
    each page's "set-visibility-map" snapshotConflictHorizon for freezing
    (at least in the vast majority of cases where freezing went ahead).
    This made VACUUM FREEZE much less prone to generating recovery conflicts
    on standbys; VACUUM FREEZE became only slightly more likely to cause
    recovery conflicts than an equivalent VACUUM.
    
    Update old documentation that specifically warned of the likelihood of
    recovery conflicts from VACUUM FREEZE.  Explain the same general issue
    (the issue of VACUUM generating recovery conflicts even in the absence
    of dead row cleanup) using the example of conflicts caused by VISIBLE
    WAL records.


 doc/src/sgml/high-availability.sgml | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/doc/src/sgml/high-availability.sgml b/doc/src/sgml/high-availability.sgml
index cf61b2ed2a1..5f9257313a2 100644
--- a/doc/src/sgml/high-availability.sgml
+++ b/doc/src/sgml/high-availability.sgml
@@ -1873,10 +1873,16 @@ synchronous_standby_names = 'ANY 2 (s1, s2, s3)'
    </para>
 
    <para>
-    Experienced users should note that both row version cleanup and row version
-    freezing will potentially conflict with standby queries. Running a manual
-    <command>VACUUM FREEZE</command> is likely to cause conflicts even on tables with
-    no updated or deleted rows.
+    Row version cleanup isn't the only potential cause of conflicts with
+    standby queries.  All index-only scans (including those that run on
+    standbys) must use an <acronym>MVCC</acronym> snapshot that
+    <quote>agrees</quote> with the visibility map.  Conflicts are therefore
+    required whenever <command>VACUUM</command> <link
+     linkend="vacuum-for-visibility-map">sets a page as all-visible in the
+     visibility map</link> containing one or more rows
+    <emphasis>not</emphasis> visible to all standby queries.  So even running
+    <command>VACUUM</command> against a table with no updated or deleted rows
+    requiring cleanup might lead to conflicts.
    </para>
 
    <para>


[parent: f7faa9976cc0]