Drop the rule against included index columns duplicating key columns.

The initial version of the included-index-column feature stated that
included columns couldn't be the same as any key column of the index.
While it'd be pretty silly to do that, since the included column would be
entirely redundant, we've never prohibited redundant index columns before
so it's not very consistent to do so here.  Moreover, the prohibition
was itself badly implemented, so that it failed to reject columns that
were effectively identical but not spelled quite alike, as reported by
Aditya Toshniwal.

(Moreover, it's not hard to imagine that for some non-btree index types,
such cases would be non-silly anyhow: the index might use a lossy
representation for key columns but be able to support retrieval of the
original form of included columns.)

Hence, let's just drop the prohibition.

In passing, do some copy-editing on the documentation for the
included-column feature.

Yugo Nagata; documentation and test corrections by me

Discussion: https://postgr.es/m/CAM9w-_mhBCys4fQNfaiQKTRrVWtoFrZ-wXmDuE9Nj5y-Y7aDKQ@mail.gmail.com

src/test/regress/expected/index_including.out

@@ -7,17 +7,17 @@
 -- Regular index with included columns
 CREATE TABLE tbl_include_reg (c1 int, c2 int, c3 int, c4 box);
 INSERT INTO tbl_include_reg SELECT x, 2*x, 3*x, box('4,4,4,4') FROM generate_series(1,10) AS x;
-CREATE INDEX tbl_include_reg_idx ON tbl_include_reg using btree (c1, c2) INCLUDE (c3,c4);
--- must fail because of intersection of key and included columns
-CREATE INDEX tbl_include_reg_idx ON tbl_include_reg using btree (c1, c2) INCLUDE (c1,c3);
-ERROR:  included columns must not intersect with key columns
+CREATE INDEX tbl_include_reg_idx ON tbl_include_reg (c1, c2) INCLUDE (c3, c4);
+-- duplicate column is pretty pointless, but we allow it anyway
+CREATE INDEX ON tbl_include_reg (c1, c2) INCLUDE (c1, c3);
 SELECT pg_get_indexdef(i.indexrelid)
 FROM pg_index i JOIN pg_class c ON i.indexrelid = c.oid
 WHERE i.indrelid = 'tbl_include_reg'::regclass ORDER BY c.relname;
-                                         pg_get_indexdef                                          
---------------------------------------------------------------------------------------------------
+                                                pg_get_indexdef                                                
+---------------------------------------------------------------------------------------------------------------
+ CREATE INDEX tbl_include_reg_c1_c2_c11_c3_idx ON public.tbl_include_reg USING btree (c1, c2) INCLUDE (c1, c3)
  CREATE INDEX tbl_include_reg_idx ON public.tbl_include_reg USING btree (c1, c2) INCLUDE (c3, c4)
-(1 row)
+(2 rows)
 
 -- Unique index and unique constraint
 CREATE TABLE tbl_include_unique1 (c1 int, c2 int, c3 int, c4 box);

src/test/regress/sql/index_including.sql

@@ -8,9 +8,9 @@
 -- Regular index with included columns
 CREATE TABLE tbl_include_reg (c1 int, c2 int, c3 int, c4 box);
 INSERT INTO tbl_include_reg SELECT x, 2*x, 3*x, box('4,4,4,4') FROM generate_series(1,10) AS x;
-CREATE INDEX tbl_include_reg_idx ON tbl_include_reg using btree (c1, c2) INCLUDE (c3,c4);
--- must fail because of intersection of key and included columns
-CREATE INDEX tbl_include_reg_idx ON tbl_include_reg using btree (c1, c2) INCLUDE (c1,c3);
+CREATE INDEX tbl_include_reg_idx ON tbl_include_reg (c1, c2) INCLUDE (c3, c4);
+-- duplicate column is pretty pointless, but we allow it anyway
+CREATE INDEX ON tbl_include_reg (c1, c2) INCLUDE (c1, c3);
 SELECT pg_get_indexdef(i.indexrelid)
 FROM pg_index i JOIN pg_class c ON i.indexrelid = c.oid
 WHERE i.indrelid = 'tbl_include_reg'::regclass ORDER BY c.relname;