Re: Minimal logical decoding on standbys

Hi Alvaro,

On 8/2/21 4:56 PM, Drouvot, Bertrand wrote:
> 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.

So you have in mind to check for XLogLogicalInfoActive() first, and if
true, then open the relation and call
RelationIsAccessibleInLogicalDecoding()?

If so, then what about also creating a new
RelationIsAccessibleWhileLogicalWalLevel() or something like this doing
the same as RelationIsAccessibleInLogicalDecoding() but without the
XLogLogicalInfoActive() check?

>> 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

Do you have a "good place" in mind?

Thanks

Bertrand