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

On Tue, Apr 28, 2020 at 11:18 AM Pavel Stehule <pavel.stehule@gmail.com> wrote:
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> út 28. 4. 2020 v 16:48 odesílatel Tomas Vondra <tomas.vondra@2ndquadrant.com> napsal:
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>> On Tue, Apr 28, 2020 at 03:43:43PM +0200, Pavel Stehule wrote:
>> >út 28. 4. 2020 v 15:26 odesílatel Tomas Vondra <tomas.vondra@2ndquadrant.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.

Yes, this is what I meant by simple expressions.

>> >I am not sure if executor knows so expression is executed as simply
>> >expressions. But probably it can be deduced from context
>> >
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>> 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'll look into doing one of these.

>> 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.
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> Theoretically it is possible, not now. But I don't think so it is necessary. I cannot to remember this pattern in any
plpgsqlcode and I never seen any request on this feature. 
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> I don't think so this is task for plpgsql engine. Maybe for JIT sometimes.

Agreed. I'd thought about this kind of scenario when I brought this
up, but I think solving it would the responsibility of the pg/pgsql
compiler rather than the expression evaluation code, because it'd have
to recognize the situation and setup a shared expression evaluation
context to be reused each time through the loop.

James