pgsql: Accept relations of any kind in LOCK TABLE

Accept relations of any kind in LOCK TABLE

The restriction that only tables and views can be locked by LOCK TABLE
is quite arbitrary, since the underlying mechanism can lock any relation
type. Drop the restriction so that programs such as pg_dump can lock
all relations they're interested in, preventing schema changes that
could cause a dump to fail after expending much effort.

Backpatch to 9.5.

Author: Álvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Reported-by: Wells Oliver <wells(dot)oliver(at)gmail(dot)com>
Discussion: https://postgr.es/m/20201021200659.GA32358@alvherre.pgsql

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/59ab4ac32460a6a93b665f4e487d7ff64979ba4d

Modified Files
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doc/src/sgml/ref/lock.sgml | 15 +++++++++------
src/backend/commands/lockcmds.c | 28 +++++++++-------------------
src/test/regress/expected/lock.out | 11 +++++++++++
src/test/regress/sql/lock.sql | 13 +++++++++++++
4 files changed, 42 insertions(+), 25 deletions(-)