Re: Minimal logical decoding on standbys

Hi Alvaro,

On 7/28/21 5:26 PM, Alvaro Herrera wrote:
> On 2021-Jul-27, Drouvot, Bertrand wrote:
>
>> diff --git a/src/backend/utils/cache/lsyscache.c b/src/backend/utils/cache/lsyscache.c
>> +bool
>> +get_rel_logical_catalog(Oid relid)
>> +{
>> + bool res;
>> + Relation rel;
>> +
>> + /* assume previously locked */
>> + rel = table_open(relid, NoLock);
>> + res = RelationIsAccessibleInLogicalDecoding(rel);
>> + table_close(rel, NoLock);
>> +
>> + return res;
>> +}
> So RelationIsAccessibleInLogicalDecoding() does a cheap check for
> wal_level which can be done without opening the table; I think this
> function should be rearranged to avoid doing that when not needed.

Thanks for looking at it.

> Also, putting this function in lsyscache.c seems somewhat wrong since
> it's not merely accessing the system caches ...
>
> I think it would be better to move this elsewhere (relcache.c, proto in
> relcache.h, perhaps call it RelationIdIsAccessibleInLogicalDecoding) and
> short-circuit for the check that can be done before opening the table.
> At least the GiST code appears to be able to call this several times per
> vacuum run, so it makes sense to short-circuit it for the fast case.
>
> ... though looking at the GiST code again I wonder if it would be more
> sensible to just stash the table's Relation pointer somewhere in the
> context structs instead of opening and closing it time and again.

That does make sense, I'll look at it.

Bertrand