|From:||Peter Eisentraut <peter(dot)eisentraut(at)2ndquadrant(dot)com>|
|Subject:||ALTER TABLE on system catalogs|
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ALTER TABLE on system catalogs is occasionally useful, for example
ALTER TABLE pg_attribute SET (autovacuum_vacuum_scale_factor=0);
However, this doesn't actually work. The above command produces
ERROR: AccessExclusiveLock required to add toast table.
If it's a shared catalog, it will produce
ERROR: shared tables cannot be toasted after initdb
In other cases it will work but then silently add a TOAST table to a
catalog, which I think we don't want.
The problem is that for (almost) any ALTER TABLE command, it afterwards
checks if the just-altered table now needs a TOAST table and tries to
add it if so, which will either fail or add a TOAST table that we don't
I propose that we instead silently ignore attempts to add TOAST tables
to shared and system catalogs after bootstrapping. This fixes the above
issues. I have attached a patch for this, and also a test that
enshrines which system catalogs are supposed to have TOAST tables.
As an alternative, I tried to modify the ALTER TABLE code to avoid the
try-to-add-TOAST-table path depending on what ALTER TABLE actions were
done, but that seemed incredibly more complicated and harder to maintain
in the long run.
(You still need allow_system_table_mods=on for all of this. Perhaps
that's also worth revisiting, but it's a separate issue.)
Peter Eisentraut http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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