pgsql: Fix assorted bugs related to identity column in partitioned tabl

Fix assorted bugs related to identity column in partitioned tables

When changing the data type of a column of a partitioned table, craft
the ALTER SEQUENCE command only once. Partitions do not have identity
sequences of their own and thus do not need a ALTER SEQUENCE command
for each partition.

Fix getIdentitySequence() to fetch the identity sequence associated
with the top-level partitioned table when a Relation of a partition is
passed to it. While doing so, translate the attribute number of the
partition into the attribute number of the partitioned table.

Author: Ashutosh Bapat <ashutosh(dot)bapat(at)enterprisedb(dot)com>
Reported-by: Alexander Lakhin <exclusion(at)gmail(dot)com>
Reviewed-by: Dmitry Dolgov <9erthalion6(at)gmail(dot)com>
Discussion: https://www.postgresql.org/message-id/3b8a9dc1-bbc7-0ef5-6863-c432afac7d59@gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/509199587df73f06eda898ae13284292f4ae573a

Modified Files
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src/backend/catalog/pg_depend.c | 31 ++++++++++++++++++--
src/backend/commands/tablecmds.c | 2 +-
src/backend/parser/parse_utilcmd.c | 52 ++++++++++++++++++++--------------
src/backend/rewrite/rewriteHandler.c | 19 +------------
src/include/catalog/dependency.h | 2 +-
src/test/regress/expected/identity.out | 32 +++++++++++++++++++--
src/test/regress/sql/identity.sql | 11 +++++--
7 files changed, 101 insertions(+), 48 deletions(-)