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
701fd0bbc9 Drop the rule against included index columns duplicating key columns.
doc/src/sgml/catalogs.sgml | 24 +++++++++------
doc/src/sgml/indexam.sgml | 9 +++---
doc/src/sgml/indices.sgml | 8 ++---
doc/src/sgml/ref/create_index.sgml | 44 ++++++++++++---------------
doc/src/sgml/ref/create_table.sgml | 10 +++---
src/backend/commands/indexcmds.c | 7 +----
src/test/regress/expected/index_including.out | 14 ++++-----
src/test/regress/sql/index_including.sql | 6 ++--
8 files changed, 58 insertions(+), 64 deletions(-)