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List:       postgresql-general
Subject:    Re: Getting "could not read block" error when creating an index on a function.
From:       Tom Lane <tgl () sss ! pgh ! pa ! us>
Date:       2020-12-30 20:14:31
Message-ID: 2402314.1609359271 () sss ! pgh ! pa ! us
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Karsten Hilbert <Karsten.Hilbert@gmx.net> writes:
> Am Wed, Dec 30, 2020 at 02:37:59PM -0500 schrieb Demitri Muna:
>> I want to index the results of these repeated, unchanging calculations =
to speed up other queries. Which mechanism would be best to do this? Creat=
e additional columns? Create another table?

> A materialized view ?

Yeah, or you might be able to do something with a before-insert-or-update
trigger that computes whatever desired value you want and fills it into a
derived column.  Indexing that column then gives the same results as
indexing the derived expression; but it sidesteps the semantic problems
because the time of computation of the expression is well-defined, even
if it's not immutable.

You might try to avoid a handwritten trigger by defining a generated
column instead, but we insist that generation expressions be immutable
so it won't really work.  (Of course, you could still lie about the
mutability of the expression, but I can't recommend that.  Per Henry
Spencer's well-known dictum, "If you lie to the compiler, it will get its
revenge".  He was speaking of C compilers, I suppose, but the principle
applies to database query optimizers too.)

			regards, tom lane


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