Re: Allowing NOT IN to use ANTI joins

On 24 June 2014 23:22, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:

>> On a more positive or even slightly exciting note I think I've managed to
>> devise a way that ANTI JOINS can be used for NOT IN much more often. It
>> seems that find_nonnullable_vars will analyse a quals list to find
>> expressions that mean that the var cannot be NULL. This means we can perform
>> ANTI JOINS for NOT IN with queries like:
>>
>> SELECT * FROM a WHERE id NOT IN(SELECT nullable_col FROM b WHERE
>> nullable_col = 1);
>> or
>> SELECT * FROM a WHERE id NOT IN(SELECT nullable_col FROM b WHERE
>> nullable_col IS NOT NULL);
>>
>> (The attached patch implements this)
>>
>> the nullable_col =1 will mean that nullable_col cannot be NULL, so the ANTI
>> JOIN can be performed safely. I think this combined with the NOT NULL check
>> will cover probably just about all valid uses of NOT IN with a subquery...
>> unless of course I've assumed something wrongly about find_nonnullable_vars.
>> I just need the correct RangeTblEntry in order to determine if the
>> TargetEntry is from an out join.
>
> This is the better way to go. It's much better to have explicit proof
> its not null than a possibly long chain of metadata that might be
> buggy.
>
>> The attached patch is a broken implemention that still needs the lookup code
>> fixed to reference the correct RTE. The failing regression tests show where
>> the problems lie.
>>
>> Any help on this would be really appreciated.
>
> I'd suggest we just drop the targetlist approach completely.

To be clearer, what I mean is we use only the direct proof approach,
for queries like this

SELECT * FROM a WHERE id NOT IN(SELECT unknown_col FROM b WHERE
unknown_col IS NOT NULL);

and we don't try to do it for queries like this

SELECT * FROM a WHERE id NOT IN(SELECT not_null_column FROM b);

because we don't know the full provenance of "not_null_column" in all
cases and that info is (currently) unreliable.

Once we have the transform working for one case, we can try to extend
the cases covered.

--
Simon Riggs http://www.2ndQuadrant.com/
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