Re: Odd pg dump error: cache lookup failure

Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> writes:
> On 2020-Oct-21, Tom Lane wrote:
>> I think the "test for that capability" bit needs more subtlety.

> Great ideas, thanks -- all adopted in the attached version. I didn't
> test this with 9.5 but as you say NOWAIT is already supported there and
> the command itself does work.

OK, looking a bit harder this time:

1. I think this bit in LockViewRecurse_walker must be removed as well:

/* Currently, we only allow plain tables or views to be locked. */
if (relkind != RELKIND_RELATION && relkind != RELKIND_PARTITIONED_TABLE &&
relkind != RELKIND_VIEW)
continue;

(Mostly, stuff in views would be ordinary tables anyway ... but
foreign tables are a possible exception.)

2. Should we add anything to the LOCK TABLE reference page? I see nothing
there now that bears on this change, but ...

3. Is hard-coding #define's for ERRCODEs actually the best we can do here?
I guess it is right now, but ugh. Seems like we ought to find a way for
frontend code to make use of errcodes.h. Not a matter for this patch
though.

4. You neglected to change the table name in the warn_or_exit_horribly
error message.

> Testing for a view as I was doing was not good, because PG12 does allow
> locks on views. I decided to use pg_aggregate_fnoid_index because it's
> easier for manual tests: it doesn't prevent connecting to the database
> when a strong lock is held. This in contrast with other more obvious
> candidates. It's a pretty arbitrary choice but it shouldn't harm
> anything since we don't actually hold the lock for more than an instant,
> and that only if it's not contended.

There might be some value in using one of pg_class's indexes instead,
mainly because the parser will surely need to touch that on the way
to performing the LOCK, while pg_aggregate is far afield. Given the
limited range of PG versions that we'll ever need to perform the check
for, I don't think there's much risk in using any well-known index
from a version-compatibility standpoint; but acquiring locks across
different catalogs could conceivably risk some issues vs a VACUUM
FULL or the like.

Other than these points, it seems committable.

regards, tom lane