Dmitry,
> > First, in the original example:
> > select u, u, u
> > from (select gen_uuid() u from rdb$database)
> >
> > there is only one instance of the expression -- there are 3 references to
> > the
> *resolved* expression.
>
> In Firebird, there is no such thing as "instance of the expression" or
> "resolved
> expression". All expression references are just copies of the original
> expression.
You said that they calculated the expression for each reference, thus the
expression can be said to be "unresolved". My point was that FB should be
"resolving"/calculating the expression once and then returning a
consistent/constant for all references for a given row.
> > P.S. This subject would extend to how the engine currently considers
> > all
> functions as "variant" (the answer changes with each call), where most
> functions/UDFs are "invariant". This causes optimization problems in cases
> where the SQL reads:
> >
> > SELECT ... FROM ... WHERE ColumnA = COS( :Param)
> >
> > In this case, the result of the COS() function is a fixed value and thus the
> optimizer should use any ColumnA index, but it doesn't.
>
> I don't get the point, sorry. Are you talking about caching invariant
> expression
> results or about index based optimizations? Predicate like "ColumnA =
> COS(:Param)" will always use an index for ColumnA regardless of whether
> COS is variant or invariant.
Bad example, I should have said that:
SELECT ... FROM ... WHERE ColumnA = COS( ColumnB)
Doesn't use an index when it should be able to, just as the engine is able to
use an index for this query
SELECT ... FROM ... WHERE ColumnA = ColumnB
Sean
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