sql: fix pg_catalog.pg_constraint's confkey column

Prior to this patch, all columns in the index were included instead of only the
ones being used in the foreign key reference.

Fixes cockroachdb#31545.

Release note (bug fix): Fix pg_catalog.pg_constraint's confkey column from
including columns that were not involved in the foreign key reference.

pkg/sql/logictest/testdata/logic_test/pg_catalog

@@ -1624,3 +1624,61 @@ query OT
 SELECT typ.oid, typ.typname FROM pg_attribute att JOIN pg_type typ ON atttypid=typ.oid WHERE attrelid='coltab'::regclass AND attname='a'
 ----
 25 text
+
+subtest 31545
+
+# Test an index of 2 referencing an index of 2.
+statement ok
+CREATE TABLE a (
+  id_a_1 INT UNIQUE,
+  id_a_2 INT,
+  PRIMARY KEY (id_a_1, id_a_2)
+)
+
+statement ok
+CREATE TABLE b (
+  id_b_1 INT,
+  id_b_2 INT,
+  PRIMARY KEY (id_b_1, id_b_2),
+  CONSTRAINT my_fkey FOREIGN KEY (id_b_1, id_b_2) REFERENCES a (id_a_1, id_a_2)
+)
+
+query TT colnames
+SELECT conkey, confkey FROM pg_catalog.pg_constraint WHERE conname = 'my_fkey'
+----
+conkey  confkey
+{1,2}   {1,2}
+
+# Test an index of 3 referencing an index of 2.
+statement ok
+DROP TABLE b;
+CREATE TABLE b (
+  id_b_1 INT,
+  id_b_2 INT,
+  id_b_3 INT,
+  PRIMARY KEY (id_b_1, id_b_2, id_b_3),
+  CONSTRAINT my_fkey FOREIGN KEY (id_b_1, id_b_2) REFERENCES a (id_a_1, id_a_2)
+)
+
+query TT colnames
+SELECT conkey, confkey FROM pg_catalog.pg_constraint WHERE conname = 'my_fkey'
+----
+conkey  confkey
+{1,2}   {1,2}
+
+# Test an index of 3 referencing an index of 1.
+statement ok
+DROP TABLE b;
+CREATE TABLE b (
+  id_b_1 INT,
+  id_b_2 INT,
+  id_b_3 INT,
+  PRIMARY KEY (id_b_1, id_b_2, id_b_3),
+  CONSTRAINT my_fkey FOREIGN KEY (id_b_1) REFERENCES a (id_a_1)
+)
+
+query TT colnames
+SELECT conkey, confkey FROM pg_catalog.pg_constraint WHERE conname = 'my_fkey'
+----
+conkey  confkey
+{1}     {1}

pkg/sql/pg_catalog.go

@@ -680,7 +680,11 @@ CREATE TABLE pg_catalog.pg_constraint (
 					confupdtype = fkActionNone
 					confdeltype = fkActionNone
 					confmatchtype = fkMatchTypeSimple
-					if conkey, err = colIDArrayToDatum(con.Index.ColumnIDs); err != nil {
+					columnIDs := con.Index.ColumnIDs
+					if con.FK.SharedPrefixLen < int32(len(columnIDs)) {
+						columnIDs = columnIDs[:con.FK.SharedPrefixLen]
+					}
+					if conkey, err = colIDArrayToDatum(columnIDs); err != nil {
 						return err
 					}
 					if confkey, err = colIDArrayToDatum(con.ReferencedIndex.ColumnIDs); err != nil {