Improving information_schema._pg_expandarray()

I happened to notice that information_schema._pg_expandarray(),
which has the nigh-unreadable definition

    AS 'select $1[s],
        s operator(pg_catalog.-) pg_catalog.array_lower($1,1) operator(pg_catalog.+) 1
        from pg_catalog.generate_series(pg_catalog.array_lower($1,1),
                                        pg_catalog.array_upper($1,1),
                                        1) as g(s)';

can now be implemented using unnest():

    AS 'SELECT * FROM pg_catalog.unnest($1) WITH ORDINALITY';

It seems to be slightly more efficient this way, but the main point
is to make it more readable.

I then realized that we could also borrow unnest's infrastructure
for rowcount estimation:

    ROWS 100 SUPPORT pg_catalog.array_unnest_support

because array_unnest_support just looks at the array argument and
doesn't have any hard dependency on the function being specifically
unnest().  I'm not sure that any of its uses in information_schema
can benefit from that right now, but surely it can't hurt.

One minor annoyance is that psql.sql is using _pg_expandarray
as a test case for \sf[+].  While we could keep doing so, I think
the main point of that test case is to exercise \sf+'s line
numbering ability, so the new one-line body is not a great test.
I changed that test to use _pg_index_position instead.

            regards, tom lane

diff --git a/src/backend/catalog/information_schema.sql b/src/backend/catalog/information_schema.sql
index 10b34c3c5b..893f73ecb5 100644
--- a/src/backend/catalog/information_schema.sql
+++ b/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],
-        s operator(pg_catalog.-) pg_catalog.array_lower($1,1) operator(pg_catalog.+) 1
-        from pg_catalog.generate_series(pg_catalog.array_lower($1,1),
-                                        pg_catalog.array_upper($1,1),
-                                        1) as g(s)';
+    ROWS 100 SUPPORT pg_catalog.array_unnest_support
+    AS 'SELECT * FROM pg_catalog.unnest($1) WITH ORDINALITY';

 /* Given an index's OID and an underlying-table column number, return the
  * column's position in the index (NULL if not there) */
diff --git a/src/backend/utils/adt/arrayfuncs.c b/src/backend/utils/adt/arrayfuncs.c
index 631012a0f2..e783a24519 100644
--- a/src/backend/utils/adt/arrayfuncs.c
+++ b/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)
diff --git a/src/test/regress/expected/psql.out b/src/test/regress/expected/psql.out
index 13e4f6db7b..5d61e4c7bb 100644
--- a/src/test/regress/expected/psql.out
+++ b/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
-CREATE OR REPLACE FUNCTION information_schema._pg_expandarray(anyarray, OUT x anyelement, OUT n integer)
- 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
-AS $function$select $1[s],
-        s operator(pg_catalog.-) pg_catalog.array_lower($1,1) operator(pg_catalog.+) 1
-        from pg_catalog.generate_series(pg_catalog.array_lower($1,1),
-                                        pg_catalog.array_upper($1,1),
-                                        1) as g(s)$function$
-\sf+ information_schema._pg_expandarray
-        CREATE OR REPLACE FUNCTION information_schema._pg_expandarray(anyarray, OUT x anyelement, OUT n integer)
-         RETURNS SETOF record
+ STABLE STRICT
+BEGIN ATOMIC
+ SELECT (ss.a).n AS n
+    FROM ( SELECT information_schema._pg_expandarray(pg_index.indkey) AS a
+            FROM pg_index
+           WHERE (pg_index.indexrelid = $1)) ss
+   WHERE ((ss.a).x = $2);
+END
+\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
-1       AS $function$select $1[s],
-2               s operator(pg_catalog.-) pg_catalog.array_lower($1,1) operator(pg_catalog.+) 1
-3               from pg_catalog.generate_series(pg_catalog.array_lower($1,1),
-4                                               pg_catalog.array_upper($1,1),
-5                                               1) as g(s)$function$
+         STABLE STRICT
+1       BEGIN ATOMIC
+2        SELECT (ss.a).n AS n
+3           FROM ( SELECT information_schema._pg_expandarray(pg_index.indkey) AS a
+4                   FROM pg_index
+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
diff --git a/src/test/regress/sql/psql.sql b/src/test/regress/sql/psql.sql
index 695c72d866..f199d624d3 100644
--- a/src/test/regress/sql/psql.sql
+++ b/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_expandarray
+\sf information_schema._pg_index_position
+\sf+ information_schema._pg_index_position
 \sf+ interval_pl_time
 \sf ts_debug(text)
 \sf+ ts_debug(text)