Re: Making Vars outer-join aware

"David G. Johnston" <david.g.johnston@gmail.com> writes:
> On Tue, Jan 24, 2023 at 12:31 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> select ... from t1 left join t2 on (t1.x = t2.y and t1.x = 1);
>>
>> If we turn the generic equivclass.c logic loose on these clauses,
>> it will deduce t2.y = 1, which is good, and then apply t2.y = 1 at
>> the scan of t2, which is even better (since we might be able to turn
>> that into an indexscan qual).  However, it will also try to apply
>> t1.x = 1 at the scan of t1, and that's just wrong, because that
>> will eliminate t1 rows that should come through with null extension.

> Is there a particular comment or README where that last conclusion is
> explained so that it makes sense.

Hm?  It's a LEFT JOIN, so it must not eliminate any rows from t1.
A row that doesn't have t1.x = 1 will appear in the output with
null columns for t2 ... but it must still appear, so we cannot
filter on t1.x = 1 in the scan of t1.