Improve the implementation of information_schema._pg_expandarray().

This function was originally coded with a handmade expansion
of the array subscripts.  We can do it a little faster and far
more legibly today, by using unnest() WITH ORDINALITY.

While at it, let's apply the rowcount estimation support that exists
for the underlying unnest() function: reduce the default ROWS estimate
to 100 and attach array_unnest_support.  I'm not sure that
array_unnest_support can do anything useful today with the call sites
that exist in information_schema, but it can't hurt, and the existing
default rowcount of 1000 is surely much too high for any of these
cases.

The psql.sql regression script is using _pg_expandarray() as a
test case for \sf+.  While we could keep doing so, the new one-line
function body makes a poor test case for \sf+ row-numbering, so
switch it to print another information_schema function.

Discussion: https://postgr.es/m/1424303.1703355485@sss.pgh.pa.us

src/backend/catalog/information_schema.sql

@@ -43,11 +43,8 @@ SET search_path TO information_schema;
 CREATE FUNCTION _pg_expandarray(IN anyarray, OUT x anyelement, OUT n int)
     RETURNS SETOF RECORD
     LANGUAGE sql STRICT IMMUTABLE PARALLEL SAFE
-    AS 'select $1[s],
+    ROWS 100 SUPPORT pg_catalog.array_unnest_support
-        s operator(pg_catalog.-) pg_catalog.array_lower($1,1) operator(pg_catalog.+) 1
+    AS 'SELECT * FROM pg_catalog.unnest($1) WITH ORDINALITY';
-        from pg_catalog.generate_series(pg_catalog.array_lower($1,1),
-                                        pg_catalog.array_upper($1,1),
-                                        1) as g(s)';
 
 /* Given an index's OID and an underlying-table column number, return the
  * column's position in the index (NULL if not there) */

src/backend/utils/adt/arrayfuncs.c

@@ -6317,6 +6317,9 @@ array_unnest(PG_FUNCTION_ARGS)
 
 /*
  * Planner support function for array_unnest(anyarray)
+ *
+ * Note: this is now also used for information_schema._pg_expandarray(),
+ * which is simply a wrapper around array_unnest().
  */
 Datum
 array_unnest_support(PG_FUNCTION_ARGS)

src/include/catalog/catversion.h

@@ -57,6 +57,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	202312251
+#define CATALOG_VERSION_NO	202312271
 
 #endif

src/test/regress/expected/psql.out

@@ -5293,26 +5293,30 @@ comment on function psql_df_plpgsql () is 'some comment';
 rollback;
 drop role regress_psql_user;
 -- check \sf
-\sf information_schema._pg_expandarray
+\sf information_schema._pg_index_position
-CREATE OR REPLACE FUNCTION information_schema._pg_expandarray(anyarray, OUT x anyelement, OUT n integer)
+CREATE OR REPLACE FUNCTION information_schema._pg_index_position(oid, smallint)
- RETURNS SETOF record
+ RETURNS integer
  LANGUAGE sql
- IMMUTABLE PARALLEL SAFE STRICT
+ STABLE STRICT
-AS $function$select $1[s],
+BEGIN ATOMIC
-        s operator(pg_catalog.-) pg_catalog.array_lower($1,1) operator(pg_catalog.+) 1
+ SELECT (ss.a).n AS n
-        from pg_catalog.generate_series(pg_catalog.array_lower($1,1),
+    FROM ( SELECT information_schema._pg_expandarray(pg_index.indkey) AS a
-                                        pg_catalog.array_upper($1,1),
+            FROM pg_index
-                                        1) as g(s)$function$
+           WHERE (pg_index.indexrelid = $1)) ss
-\sf+ information_schema._pg_expandarray
+   WHERE ((ss.a).x = $2);
-        CREATE OR REPLACE FUNCTION information_schema._pg_expandarray(anyarray, OUT x anyelement, OUT n integer)
+END
-         RETURNS SETOF record
+\sf+ information_schema._pg_index_position
+        CREATE OR REPLACE FUNCTION information_schema._pg_index_position(oid, smallint)
+         RETURNS integer
          LANGUAGE sql
-         IMMUTABLE PARALLEL SAFE STRICT
+         STABLE STRICT
-1       AS $function$select $1[s],
+1       BEGIN ATOMIC
-2               s operator(pg_catalog.-) pg_catalog.array_lower($1,1) operator(pg_catalog.+) 1
+2        SELECT (ss.a).n AS n
-3               from pg_catalog.generate_series(pg_catalog.array_lower($1,1),
+3           FROM ( SELECT information_schema._pg_expandarray(pg_index.indkey) AS a
-4                                               pg_catalog.array_upper($1,1),
+4                   FROM pg_index
-5                                               1) as g(s)$function$
+5                  WHERE (pg_index.indexrelid = $1)) ss
+6          WHERE ((ss.a).x = $2);
+7       END
 \sf+ interval_pl_time
         CREATE OR REPLACE FUNCTION pg_catalog.interval_pl_time(interval, time without time zone)
          RETURNS time without time zone

src/test/regress/sql/psql.sql

@@ -1312,8 +1312,8 @@ rollback;
 drop role regress_psql_user;
 
 -- check \sf
-\sf information_schema._pg_expandarray
+\sf information_schema._pg_index_position
-\sf+ information_schema._pg_expandarray
+\sf+ information_schema._pg_index_position
 \sf+ interval_pl_time
 \sf ts_debug(text)
 \sf+ ts_debug(text)