I have been searching through the docs and mailing list and haven't found a way to do this, so I thought I would ask the community.
I would like to know if there is a way in PostgreSQL to avoid repeating an expensive computation in a SELECT where the result is needed both as a returned value and as an expression in the WHERE clause.
As a simple example, consider the following query on a table with 'id' and 'value' columns, and an expensive computation represented as a function:
SELECT id, expensivefunc(value) AS score FROM mytable
WHERE id LIKE '%z%' AND expensivefunc(value) > 0.5;
It would be great if I could find a way to only compute expensivefunc(value) at most once per row, and not at all if the other WHERE constraints are not satisfied.
For this simple case I know that I could rewrite the SELECT as something like the following:
WITH other_where AS (
SELECT id, value FROM mytable WHERE id LIKE '%z%'
), calc_scores AS (
SELECT id, expensivefunc(value) AS score FROM other_where
)
SELECT id, score from calc_scores WHERE score > 0.5;
This works in this simple case, but my guess is that it probably adds a lot of overhead (is this true?), and I also have to deal with much more complicated scenarios with multiple expensive calculations that may not fit into this kind of rewrite.
Does anyone know of a simpler way to accomplish this?
For example, it would be great if there were a function that could reference the Nth select list item so it is only computed once, like:
SELECT id, expensivefunc(value) AS score FROM mytable
WHERE id LIKE '%z%' AND sel_list_item(2) > 0.5;
or if there were temporary variables in the WHERE expressions like:
SELECT id, tmp1 AS score FROM mytable
WHERE id LIKE '%z%' AND (tmp1 = expensivefunc(value)) > 0.5;
Any ideas anyone!
Thanks in advance!
Bob
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