S1:
rhaas=# create table foo (a int);
CREATE TABLE
rhaas=# begin;
BEGIN
rhaas=# lock foo;
LOCK TABLE
S2:
rhaas=# grant all on foo to public;
GRANT
rhaas=# revoke all on foo from public;
REVOKE
This seems quite obviously silly, given the amount of time and energy
we've spent worrying about ALTER TABLE lock levels. Note that
GRANT/REVOKE on a table do a not-in-place update of the pg_class row;
with anything less than an AccessExclusiveLock, the usual SnapshotNow
hazards exist: another session can fail to find the pg_class row
altogether.
[ Credit: Noah Misch helped me trace down the problem that led me to
this report. ]
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company