Re: [HACKERS] Runtime Partition Pruning

On Thu, Dec 21, 2017 at 6:53 PM, David Rowley
<david(dot)rowley(at)2ndquadrant(dot)com> wrote:
> On 22 December 2017 at 12:45, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>> PREPARE q3 (int, int) AS SELECT * FROM unpartitioned_table WHERE
>> partkey BETWEEN $1 AND $2;
>>
>> ...that has to decide whether to use an index. And to do that it has
>> to estimate what fraction of the table will match the BETWEEN clause.
>> That may be an uninformed guess, but it guesses something. We
>> probably want to do something here that makes the guess for a
>> partitioned_table similar to the guess for an unpartitioned_table.
>
> Are you considering some sort of clauselist_selectivity() estimate on
> the given parameters and comparing that to the same selectivities that
> were determined for the previous custom plans?

No, I don't think comparing to previous custom plans is a workable
approach. I was thinking, rather, that if we know for example that
we've doing pruning on partition_column = $1, then we know that only
one partition will match. That's probably a common case. If we've
got partition_column > $1, we could assume that, say, 75% of the
partitions would match. partition_column BETWEEN $1 and $2 is
probably a bit more selective, so maybe we assume 50% of the
partitions would match.

--
Robert Haas
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