Re: [HACKERS] Adding support for Default partition in partitioning - Mailing list pgsql-hackers

On Wed, Sep 6, 2017 at 5:50 PM, Jeevan Ladhe
<jeevan.ladhe@enterprisedb.com> wrote:
>
>>
>> I am wondering whether we could avoid call to get_default_partition_oid()
>> in
>> the above block, thus avoiding a sys cache lookup. The sys cache search
>> shouldn't be expensive since the cache should already have that entry, but
>> still if we can avoid it, we save some CPU cycles. The default partition
>> OID is
>> stored in pg_partition_table catalog, which is looked up in
>> RelationGetPartitionKey(), a function which precedes
>> RelationGetPartitionDesc()
>> everywhere. What if that RelationGetPartitionKey() also returns the
>> default
>> partition OID and the common caller passes it to
>> RelationGetPartitionDesc()?.
>
>
> The purpose here is to cross check the relid with partdefid stored in
> catalog
> pg_partitioned_table, though its going to be the same in the parents cache,
> I
> think its better that we retrieve it from the catalog syscache.
> Further, RelationGetPartitionKey() is a macro and not a function, so
> modifying
> the existing simple macro for this reason does not sound a good idea to me.
> Having said this I am open to ideas here.

Sorry, I meant RelationBuildPartitionKey() and
RelationBuildPartitionDesc() instead of RelationGetPartitionKey() and
RelationGetPartitionDesc() resp.

>
>>
>> +    /* A partition cannot be attached if there exists a default partition
>> */
>> +    defaultPartOid = get_default_partition_oid(RelationGetRelid(rel));
>> +    if (OidIsValid(defaultPartOid))
>> +        ereport(ERROR,
>> +                (errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
>> +                 errmsg("cannot attach a new partition to table
>> \"%s\" having a default partition",
>> +                        RelationGetRelationName(rel))));
>> get_default_partition_oid() searches the catalogs, which is not needed
>> when we
>> have relation descriptor of the partitioned table (to which a new
>> partition is
>> being attached). You should get the default partition OID from partition
>> descriptor. That will be cheaper.
>
>
> Something like following can be done here:
>     /* A partition cannot be attached if there exists a default partition */
>     if (partition_bound_has_default(rel->partdesc->boundinfo))
>         ereport(ERROR,
>                 (errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
>                  errmsg("cannot attach a new partition to table \"%s\"
> having a default partition",
>                         RelationGetRelationName(rel))));
>
> But, partition_bound_has_default() is defined in partition.c and not in
> partition.h. This is done that way because boundinfo is not available in
> partition.h. Further, this piece of code is removed in next patch where we
> extend default partition support to add/attach partition even when default
> partition exists. So, to me I don’t see much of the correction issue here.

If the code is being removed, I don't think we should sweat too much
about it. Sorry for the noise.

>
> Another way to get around this is, we can define another version of
> get_default_partition_oid() something like
> get_default_partition_oid_from_parent_rel()
> in partition.c which looks around in relcache instead of catalog and returns
> the
> oid of default partition, or get_default_partition_oid() accepts both parent
> OID,
> and parent ‘Relation’ rel, if rel is not null look into relcahce and return,
> else search from catalog using OID.

I think we should define a function to return default partition OID
from partition descriptor and make it extern. Define a wrapper which
accepts Relation and returns calls this function to get default
partition OID from partition descriptor. The wrapper will be called
only on an open Relation, wherever it's available.


>
>> I haven't gone through the full patch yet, so there may be more
>> comments here. There is some duplication of code in
>> check_default_allows_bound() and ValidatePartitionConstraints() to
>> scan the children of partition being validated. The difference is that
>> the first one scans the relations in the same function and the second
>> adds them to work queue. May be we could use
>> ValidatePartitionConstraints() to scan the relation or add to the
>> queue based on some input flag may be wqueue argument itself. But I
>> haven't thought through this completely. Any thoughts?
>
>
> check_default_allows_bound() is called only from DefineRelation(),
> and not for alter command. I am not really sure how can we use
> work queue for create command.


No, we shouldn't use work queue for CREATE command. We should extract
the common code into a function to be called from
check_default_allows_bound() and ValidatePartitionConstraints(). To
that function we pass a flag (or the work queue argument itself),
which decides whether to add a work queue item or scan the relation
directly.

--
Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company