Enhance nbtree ScalarArrayOp execution.

Commit 9e8da0f taught nbtree to handle ScalarArrayOpExpr quals
natively.  This works by pushing down the full context (the array keys)
to the nbtree index AM, enabling it to execute multiple primitive index
scans that the planner treats as one continuous index scan/index path.
This earlier enhancement enabled nbtree ScalarArrayOp index-only scans.
It also allowed scans with ScalarArrayOp quals to return ordered results
(with some notable restrictions, described further down).

Take this general approach a lot further: teach nbtree SAOP index scans
to decide how to execute ScalarArrayOp scans (when and where to start
the next primitive index scan) based on physical index characteristics.
This can be far more efficient.  All SAOP scans will now reliably avoid
duplicative leaf page accesses (just like any other nbtree index scan).
SAOP scans whose array keys are naturally clustered together now require
far fewer index descents, since we'll reliably avoid starting a new
primitive scan just to get to a later offset from the same leaf page.

The scan's arrays now advance using binary searches for the array
element that best matches the next tuple's attribute value.  Required
scan key arrays (i.e. arrays from scan keys that can terminate the scan)
ratchet forward in lockstep with the index scan.  Non-required arrays
(i.e. arrays from scan keys that can only exclude non-matching tuples)
"advance" without the process ever rolling over to a higher-order array.

Naturally, only required SAOP scan keys trigger skipping over leaf pages
(non-required arrays cannot safely end or start primitive index scans).
Consequently, even index scans of a composite index with a high-order
inequality scan key (which we'll mark required) and a low-order SAOP
scan key (which we won't mark required) now avoid repeating leaf page
accesses -- that benefit isn't limited to simpler equality-only cases.
In general, all nbtree index scans now output tuples as if they were one
continuous index scan -- even scans that mix a high-order inequality
with lower-order SAOP equalities reliably output tuples in index order.
This allows us to remove a couple of special cases that were applied
when building index paths with SAOP clauses during planning.

Bugfix commit 807a40c taught the planner to avoid generating unsafe
path keys: path keys on a multicolumn index path, with a SAOP clause on
any attribute beyond the first/most significant attribute.  These cases
are now all safe, so we go back to generating path keys without regard
for the presence of SAOP clauses (just like with any other clause type).
Affected queries can now exploit scan output order in all the usual ways
(e.g., certain "ORDER BY ... LIMIT n" queries can now terminate early).

Also undo changes from follow-up bugfix commit a4523c5, which taught
the planner to produce alternative index paths, with path keys, but
without low-order SAOP index quals (filter quals were used instead).
We'll no longer generate these alternative paths, since they can no
longer offer any meaningful advantages over standard index qual paths.
Affected queries thereby avoid all of the disadvantages that come from
using filter quals within index scan nodes.  They can avoid extra heap
page accesses from using filter quals to exclude non-matching tuples
(index quals will never have that problem).  They can also skip over
irrelevant sections of the index in more cases (though only when nbtree
determines that starting another primitive scan actually makes sense).

There is a theoretical risk that removing restrictions on SAOP index
paths from the planner will break compatibility with amcanorder-based
index AMs maintained as extensions.  Such an index AM could have the
same limitations around ordered SAOP scans as nbtree had up until now.
Adding a pro forma incompatibility item about the issue to the Postgres
17 release notes seems like a good idea.

Author: Peter Geoghegan <[email protected]>
Author: Matthias van de Meent <[email protected]>
Reviewed-By: Heikki Linnakangas <[email protected]>
Reviewed-By: Matthias van de Meent <[email protected]>
Reviewed-By: Tomas Vondra <[email protected]>
Discussion: https://postgr.es/m/CAH2-Wz=ksvN_sjcnD1+Bt-WtifRA5ok48aDYnq3pkKhxgMQpcw@mail.gmail.com

Author: petergeoghegan

doc/src/sgml/indexam.sgml

@@ -809,14 +809,22 @@ amrestrpos (IndexScanDesc scan);
   <para>
 <programlisting>
 Size
-amestimateparallelscan (void);
+amestimateparallelscan (int nkeys,
+                        int norderbys);
 </programlisting>
    Estimate and return the number of bytes of dynamic shared memory which
    the access method will be needed to perform a parallel scan.  (This number
    is in addition to, not in lieu of, the amount of space needed for
    AM-independent data in <structname>ParallelIndexScanDescData</structname>.)
   </para>
 
+  <para>
+   The <literal>nkeys</literal> and <literal>norderbys</literal>
+   parameters indicate the number of quals and ordering operators that will be
+   used in the scan; the same values will be passed to <function>amrescan</function>.
+   Note that the actual values of the scan keys aren't provided yet.
+  </para>
+
   <para>
    It is not necessary to implement this function for access methods which
    do not support parallel scans or for which the number of additional bytes

doc/src/sgml/monitoring.sgml

@@ -4064,6 +4064,19 @@ description | Waiting for a newly initialized WAL file to reach durable storage
    </para>
   </note>
 
+  <note>
+   <para>
+    Queries that use certain <acronym>SQL</acronym> constructs to search for
+    rows matching any value out of a list or array of multiple scalar values
+    (see <xref linkend="functions-comparisons"/>) perform multiple
+    <quote>primitive</quote> index scans (up to one primitive scan per scalar
+    value) during query execution.  Each internal primitive index scan
+    increments <structname>pg_stat_all_indexes</structname>.<structfield>idx_scan</structfield>,
+    so it's possible for the count of index scans to significantly exceed the
+    total number of index scan executor node executions.
+   </para>
+  </note>
+
  </sect2>
 
  <sect2 id="monitoring-pg-statio-all-tables-view">

src/backend/access/index/indexam.c

@@ -449,13 +449,10 @@ index_restrpos(IndexScanDesc scan)
 
 /*
  * index_parallelscan_estimate - estimate shared memory for parallel scan
- *
- * Currently, we don't pass any information to the AM-specific estimator,
- * so it can probably only return a constant.  In the future, we might need
- * to pass more information.
  */
 Size
-index_parallelscan_estimate(Relation indexRelation, Snapshot snapshot)
+index_parallelscan_estimate(Relation indexRelation, int nkeys, int norderbys,
+							Snapshot snapshot)
 {
 	Size		nbytes;
 
@@ -474,7 +471,8 @@ index_parallelscan_estimate(Relation indexRelation, Snapshot snapshot)
 	 */
 	if (indexRelation->rd_indam->amestimateparallelscan != NULL)
 		nbytes = add_size(nbytes,
-						  indexRelation->rd_indam->amestimateparallelscan());
+						  indexRelation->rd_indam->amestimateparallelscan(nkeys,
+																		  norderbys));
 
 	return nbytes;
 }