Re: Declarative partitioning - another take - Mailing list pgsql-hackers

On 2016/09/22 19:10, Ashutosh Bapat wrote:
> On Thu, Sep 22, 2016 at 1:02 PM, Ashutosh Bapat
> <ashutosh.bapat@enterprisedb.com> wrote:
>> For list partitions, the ListInfo stores the index maps for values
>> i.e. the index of the partition to which the value belongs. Those
>> indexes are same as the indexes in partition OIDs array and come from
>> the catalogs. In case a user creates two partitioned tables with
>> exactly same lists for partitions but specifies them in a different
>> order, the OIDs are stored in the order specified. This means that
>> index array for these tables come out different. equal_list_info()
>> works around that by creating an array of mappings and checks whether
>> that mapping is consistent for all values. This means we will create
>> the mapping as many times as equal_list_info() is called, which is
>> expected to be more than the number of time
>> RelationBuildPartitionDescriptor() is called. Instead, if we
>> "canonicalise" the indexes so that they come out exactly same for
>> similarly partitioned tables, we build the mapping only once and
>> arrange OIDs accordingly.
>>
>> Here's patch to do that. I have ran make check with this and it didn't
>> show any failure. Please consider this to be included in your next set
>> of patches.

Thanks.  It seems like this will save quite a few cycles for future users
of equal_list_info().  I will incorporate it into may patch.

With this patch, the mapping is created *only once* during
RelationBuildPartitionDesc() to assign canonical indexes to individual
list values.  The partition OID array will also be rearranged such that
using the new (canonical) index instead of the old
catalog-scan-order-based index will retrieve the correct partition for
that value.

By the way, I fixed one thinko in your patch as follows:

-        result->oids[i] = oids[mapping[i]];
+        result->oids[mapping[i]] = oids[i];

That is, copy an OID at a given position in the original array to a
*mapped position* in the result array (instead of the other way around).

> The patch has an if condition as statement by itself
> +    if (l1->null_index != l2->null_index);
>          return false;
> 
> There shouldn't be ';' at the end. It looks like in the tests you have
> added the function always bails out before it reaches this statement.

There is no user of equal_list_info() such that the above bug would have
caused a regression test failure.  Maybe a segfault crash (due to dangling
pointer into partition descriptor's listinfo after the above function
would unintentionally return false to equalPartitionDescs() which is in
turn called by RelationClearRelation()).

Thanks,
Amit