Pass down "logically unchanged index" hint.

Add an executor aminsert() hint mechanism that informs index AMs that
the incoming index tuple (the tuple that accompanies the hint) is not
being inserted by execution of an SQL statement that logically modifies
any of the index's key columns.

The hint is received by indexes when an UPDATE takes place that does not
apply an optimization like heapam's HOT (though only for indexes where
all key columns are logically unchanged).  Any index tuple that receives
the hint on insert is expected to be a duplicate of at least one
existing older version that is needed for the same logical row.  Related
versions will typically be stored on the same index page, at least
within index AMs that apply the hint.

Recognizing the difference between MVCC version churn duplicates and
true logical row duplicates at the index AM level can help with cleanup
of garbage index tuples.  Cleanup can intelligently target tuples that
are likely to be garbage, without wasting too many cycles on less
promising tuples/pages (index pages with little or no version churn).

This is infrastructure for an upcoming commit that will teach nbtree to
perform bottom-up index deletion.  No index AM actually applies the hint
just yet.

Author: Peter Geoghegan <pg@bowt.ie>
Reviewed-By: Victor Yegorov <vyegorov@gmail.com>
Discussion: https://postgr.es/m/CAH2-Wz=CEKFa74EScx_hFVshCOn6AA5T-ajFASTdzipdkLTNQQ@mail.gmail.com

1 files changed, 15 insertions, 0 deletions

diff --git a/doc/src/sgml/indexam.sgml b/doc/src/sgml/indexam.sgml
index f00268d5b51..ec5741df6d1 100644
--- a/doc/src/sgml/indexam.sgml
+++ b/doc/src/sgml/indexam.sgml
@@ -293,6 +293,7 @@ aminsert (Relation indexRelation,
           ItemPointer heap_tid,
           Relation heapRelation,
           IndexUniqueCheck checkUnique,
+          bool indexUnchanged,
           IndexInfo *indexInfo);
 </programlisting>
    Insert a new tuple into an existing index.  The <literal>values</literal> and
@@ -309,6 +310,20 @@ aminsert (Relation indexRelation,
   </para>
 
   <para>
+   The <literal>indexUnchanged</literal> boolean value gives a hint
+   about the nature of the tuple to be indexed.  When it is true,
+   the tuple is a duplicate of some existing tuple in the index.  The
+   new tuple is a logically unchanged successor MVCC tuple version.  This
+   happens when an <command>UPDATE</command> takes place that does not
+   modify any columns covered by the index, but nevertheless requires a
+   new version in the index.  The index AM may use this hint to decide
+   to apply bottom-up index deletion in parts of the index where many
+   versions of the same logical row accumulate.  Note that updating a
+   non-key column does not affect the value of
+   <literal>indexUnchanged</literal>.
+  </para>
+
+  <para>
    The function's Boolean result value is significant only when
    <literal>checkUnique</literal> is <literal>UNIQUE_CHECK_PARTIAL</literal>.
    In this case a true result means the new entry is known unique, whereas