pgsql: ALTER TABLE ... ALTER COLUMN ... DROP EXPRESSION

ALTER TABLE ... ALTER COLUMN ... DROP EXPRESSION

Add an ALTER TABLE subcommand for dropping the generated property from
a column, per SQL standard.

Reviewed-by: Sergei Kornilov <sk@zsrv.org>
Discussion: https://www.postgresql.org/message-id/flat/2f7f1d9c-946e-0453-d841-4f38eb9d69b6%402ndquadrant.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f595117e24a79db6072979ab5a757431fd17232f

Modified Files
--------------
doc/src/sgml/ref/alter_table.sgml       |  18 ++++
src/backend/catalog/sql_features.txt    |   2 +-
src/backend/commands/tablecmds.c        | 161 +++++++++++++++++++++++++++++++-
src/backend/parser/gram.y               |  20 +++-
src/bin/psql/tab-complete.c             |   2 +-
src/include/nodes/parsenodes.h          |   1 +
src/include/parser/kwlist.h             |   1 +
src/test/regress/expected/generated.out |  87 +++++++++++++++++
src/test/regress/sql/generated.sql      |  38 ++++++++
9 files changed, 326 insertions(+), 4 deletions(-)