On 1/3/2024 22:33, Alexander Korotkov wrote:
> I'm going to review and revise the patch.
Nice!
>
> One question I have yet.
>
> > /*
> > * Transformation only works with both side type is not
> > * { array | composite | domain | record }.
>
> Why do we limit transformation for these types? Also, it doesn't seem
> the current code restricts anything except composite/record.
Answer can be a bit long. Let's try to see comment a4424c5 at first.
We forbid record types although they can have typarray. It is because of
the RowExpr comparison specific. Although we have the record_eq()
routine, all base types in comparing records must be strictly the same.
Let me show:
explain analyze
SELECT * FROM
(SELECT ROW(relpages,relnatts) AS x FROM pg_class LIMIT 10) AS q1,
(SELECT ROW(relpages,relallvisible) AS x FROM pg_class LIMIT 10) AS q2
WHERE q1.x=q2.x;
ERROR: cannot compare dissimilar column types smallint and integer at
record column 2
As you can see, we have smallint and integer in the second position of
RowExpr and it causes the ERROR. It is the reason, why PostgreSQL
transforms ROW expressions to the series of ORs, Look:
explain (costs off)
SELECT oid,relname FROM pg_class
WHERE (oid,relname) IN ((1, 'a'), (2,'b'));
Bitmap Heap Scan on pg_class
Recheck Cond: ((relname = 'a'::name) OR (relname = 'b'::name))
Filter: (((oid = '1'::oid) AND (relname = 'a'::name)) OR ((oid =
'2'::oid) AND (relname = 'b'::name)))
-> BitmapOr
...
So, transforming composite types to the ScalarArrayOpExpr expression
doesn't make sense. Am I wrong?
The same with domain. If it have composite base type we reject the
transformation according to the logic above.
What about arrays? As I see, arrays don't have typarray and we can avoid
to spend more cycles after detection of TYPCATEGORY_ARRAY. I haven't
done it yet because have a second thought: what if to combine arrays
into the larger one? I'm unsure on that, so we can forbid it too.
--
regards,
Andrei Lepikhov
Postgres Professional
