Show index search count in EXPLAIN ANALYZE, take 2.

Expose the count of index searches/index descents in EXPLAIN ANALYZE's
output for index scan/index-only scan/bitmap index scan nodes.  This
information is particularly useful with scans that use ScalarArrayOp
quals, where the number of index searches can be unpredictable due to
implementation details that interact with physical index characteristics
(at least with nbtree SAOP scans, since Postgres 17 commit 5bf748b8).
The information shown also provides useful context when EXPLAIN ANALYZE
runs a plan with an index scan node that successfully applied the skip
scan optimization (set to be added to nbtree by an upcoming patch).

The instrumentation works by teaching all index AMs to increment a new
nsearches counter whenever a new index search begins.  The counter is
incremented at exactly the same point that index AMs already increment
the pg_stat_*_indexes.idx_scan counter (we're counting the same event,
but at the scan level rather than the relation level).  Parallel queries
have workers copy their local counter struct into shared memory when an
index scan node ends -- even when it isn't a parallel aware scan node.
An earlier version of this patch that only worked with parallel aware
scans became commit 5ead85fb (though that was quickly reverted by commit
d00107cd following "debug_parallel_query=regress" buildfarm failures).

Our approach doesn't match the approach used when tracking other index
scan related costs (e.g., "Rows Removed by Filter:").  It is comparable
to the approach used in similar cases involving costs that are only
readily accessible inside an access method, not from the executor proper
(e.g., "Heap Blocks:" output for a Bitmap Heap Scan, which was recently
enhanced to show per-worker costs by commit 5a1e6df3, using essentially
the same scheme as the one used here).  It is necessary for index AMs to
have direct responsibility for maintaining the new counter, since the
counter might need to be incremented multiple times per amgettuple call
(or per amgetbitmap call).  But it is also necessary for the executor
proper to manage the shared memory now used to transfer each worker's
counter struct to the leader.

Author: Peter Geoghegan <[email protected]>
Reviewed-By: Robert Haas <[email protected]>
Reviewed-By: Tomas Vondra <[email protected]>
Reviewed-By: Masahiro Ikeda <[email protected]>
Reviewed-By: Matthias van de Meent <[email protected]>
Discussion: https://postgr.es/m/CAH2-WzkRqvaqR2CTNqTZP0z6FuL4-3ED6eQB0yx38XBNj1v-4Q@mail.gmail.com
Discussion: https://postgr.es/m/CAH2-Wz=PKR6rB7qbx+Vnd7eqeB5VTcrW=iJvAsTsKbdG+kW_UA@mail.gmail.com

1 files changed, 2 insertions, 0 deletions

diff --git a/doc/src/sgml/rules.sgml b/doc/src/sgml/rules.sgml
index 1d9924a2a3c..8467d961fd0 100644
--- a/doc/src/sgml/rules.sgml
+++ b/doc/src/sgml/rules.sgml
@@ -1046,6 +1046,7 @@ SELECT count(*) FROM words WHERE word = 'caterpiler';
    ->  Index Only Scan using wrd_word on wrd  (cost=0.42..4.44 rows=1 width=0) (actual time=0.039..0.039 rows=0.00 loops=1)
          Index Cond: (word = 'caterpiler'::text)
          Heap Fetches: 0
+         Index Searches: 1
  Planning time: 0.164 ms
  Execution time: 0.117 ms
 </programlisting>
@@ -1090,6 +1091,7 @@ SELECT word FROM words ORDER BY word &lt;-&gt; 'caterpiler' LIMIT 10;
  Limit  (cost=0.29..1.06 rows=10 width=10) (actual time=187.222..188.257 rows=10.00 loops=1)
    -&gt;  Index Scan using wrd_trgm on wrd  (cost=0.29..37020.87 rows=479829 width=10) (actual time=187.219..188.252 rows=10.00 loops=1)
          Order By: (word &lt;-&gt; 'caterpiler'::text)
+         Index Searches: 1
  Planning time: 0.196 ms
  Execution time: 198.640 ms
 </programlisting>