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help / color / mirror / Atom feedFrom: Justin Christensen <[email protected]>
To: David Rowley <[email protected]>
Cc: [email protected]
Subject: Re: Surprising behavior with pushing predicates down into a view
Date: Tue, 31 Mar 2026 11:00:09 -0500
Message-ID: <CAH+MXdy3JJjtH1=AaWO2JcF1SHJk3v2AvnhSPyr4xfWB8cORFA@mail.gmail.com> (raw)
In-Reply-To: <CAApHDvpCn2t07ag0Bh+P66pGMJj=u97ROBetTHyj71byMvcYAg@mail.gmail.com>
References: <CAH+MXdxJusXfQoyzF3x8U=nh+QxmfiqGxZ1kPYXTfd8Srx5ZHg@mail.gmail.com>
<CAApHDvpCn2t07ag0Bh+P66pGMJj=u97ROBetTHyj71byMvcYAg@mail.gmail.com>
Ah. I did happen to skim the postgres source and I did see that subplans
were being rejected in qual_is_pushdown_safe before I sent this over, and I
noticed a few comments that made it seem like that was a design choice at
the time that could stand to be revisited later: "XXX that could stand to
be reconsidered, now that we use Paths."
Is this a potentially desirable enhancement for it to work the way I
expected or is it an explicit design choice for it to remain as-is? Correct
me if I'm wrong, but it theoretically could be pushed down into the sub
query without affecting correctness, and it's just a question of effort and
query planning efficiency? Are we just saying the juice is not worth the
squeeze and making this change could affect stability?
On Tue, Mar 31, 2026 at 7:44 AM David Rowley <[email protected]> wrote:
> On Tue, 31 Mar 2026 at 22:45, Justin Christensen
> <[email protected]> wrote:
> > When I query this view using a simple constant ticker like 'AAPL' the
> query plan shows that it correctly filters the set of tickers before
> applying the return calculations, joining, and then calculating beta. I've
> included the EXPLAIN output after the query in this gist.
> >
> >
> https://gist.github.com/JustinChristensen/1cad256ea61ad30aa39d42aebc4557b9#file-literal-filter-sql
> >
> > When I query this view using a subquery to find the tickers to filter on
> it instead tries to execute the view and calculate the beta for all of the
> tickers in the table before filtering:
>
> In short, this isn't a bug.
>
> When planning subqueries, which is effectively how a VIEW will be
> planned unless it passed is_simple_subquery()'s tests, we only
> consider pushing down "base" quals into that subquery. When you do
> ticker = 'AAPL', that's a base qual, and that can be pushed down
> because ticker is in the WINDOW's PARTITION BY clause, but ticker IN
> (SELECT ticker FROM metrics ORDER BY random() LIMIT 50) is converted
> into a SEMI join much earlier in planning, so that isn't a base qual
> anymore. Even if we didn't do that tranformation in
> convert_ANY_sublink_to_join(), we'd still fail to push down the base
> qual into the subquery as qual_is_pushdown_safe() doesn't accept base
> quals with subplans.
>
> You might be better off changing the view to a table returning
> function which accepts a ticker parameter and calling that function
> once for each ticker you need.
>
> David
>
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Subject: Re: Surprising behavior with pushing predicates down into a view
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