Re: Binary search in ScalarArrayOpExpr for OR'd constant arrays

út 28. 4. 2020 v 16:48 odesílatel Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>
napsal:

> On Tue, Apr 28, 2020 at 03:43:43PM +0200, Pavel Stehule wrote:
> >út 28. 4. 2020 v 15:26 odesílatel Tomas Vondra <
> tomas(dot)vondra(at)2ndquadrant(dot)com>
> >napsal:
> >
> >> ...
> >>
> >> >I'm not so concerned about this in any query where we have a real FROM
> >> >clause because even if we end up with only one row, the relative
> >> >penalty is low, and the potential gain is very high. But simple
> >> >expressions in pl/pgsql, for example, are a case where we can know for
> >> >certain (correct me if I've wrong on this) that we'll only execute the
> >> >expression once, which means there's probably always a penalty for
> >> >choosing the implementation with setup costs over the default linear
> >> >scan through the array.
> >> >
> >>
> >> What do you mean by "simple expressions"? I'm not plpgsql expert and I
> >> see it mostly as a way to glue together SQL queries, but yeah - if we
> >> know a given ScalarArrayOpExpr will only be executed once, then we can
> >> disable this optimization for now.
> >>
> >
> >a := a + 1
> >
> >is translated to
> >
> >SELECT $1 + 1 and save result to var a
> >
> >The queries like this "SELECT $1 + 1" are simple expressions. They are
> >evaluated just on executor level - it skip SPI
> >
> >the simple expression has not FROM clause, and have to return just one
> row.
> >I am not sure if it is required, it has to return just one column.
> >
> >I am not sure if executor knows so expression is executed as simply
> >expressions. But probably it can be deduced from context
> >
>
> Not sure. The executor state is created by exec_eval_simple_expr, which
> calls ExecInitExprWithParams (and it's the only caller). And that in
> turn is the only place that leaves (state->parent == NULL). So maybe
> that's a way to identify simple (standalone) expressions? Otherwise we
> might add a new EEO_FLAG_* to identify these expressions explicitly.
>
> I wonder if it would be possible to identify cases when the expression
> is executed in a loop, e.g. like this:
>
> FOR i IN 1..1000 LOOP
> x := y IN (1, 2, ..., 999);
> END LOOP;
>
> in which case we only build the ScalarArrayOpExpr once, so maybe we
> could keep the hash table for all executions. But maybe that's not
> possible or maybe it's pointless for other reasons. It sure looks a bit
> like trying to build a query engine from FOR loop.
>

Theoretically it is possible, not now. But I don't think so it is
necessary. I cannot to remember this pattern in any plpgsql code and I
never seen any request on this feature.

I don't think so this is task for plpgsql engine. Maybe for JIT sometimes.

> regards
>
> --
> Tomas Vondra http://www.2ndQuadrant.com
> PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
>