Re: Index on immutable function call

Dean Rasheed <dean(dot)a(dot)rasheed(at)googlemail(dot)com> writes:
> 2010/1/19 Philippe Lang <philippe(dot)lang(at)attiksystem(dot)ch>:
>> That works just fine, but is there maybe a way of creating a slighly
>> more "generic" index? If I change the ">" with a "<" in the query, index
>> cannot of course be used. According to documentation, answer seems to be
>> "no"...

> You could create an index on the difference:
> create index long_transformation_index on indexed_table
> ((data1-this_is_a_long_transformation(data2)));

> then rewrite your queries accordingly:
> ... WHERE data1-this_is_a_long_transformation(data2) > some const (or
> < some const)

Yeah. There's been a lot of noise in this thread about statistics,
but the real problem is this: an indexable condition compares the
indexed value to a constant (or at least something that doesn't
change across rows). "col1 > foo(col2)" is not able to use an index
on col1, nor an index on foo(col2). But if you indexed the whole
expression col1 - foo(col2), you could get an index search on
comparisons of that to a constant.

regards, tom lane