Greetings!
I'm having some troubles creating a query, or rather, I can write one
that works but the approach feels wrong! The problem at hand boils
down to finding a record in a group where each result of two result-
sets matches on some columns.
The actual data I need to match isn't directly from tables but both
sides of the equation are the results of a set-returning function that
breaks up a unit string into separate tokens (base-unit & exponent).
An example of the two sets I need to "join" are, at the left hand side:
unit | token | exponent
-------+-------+----------
m.s^-1 | m | 1
m.s^-1 | s | -1
m.s^-2 | m | 1
m.s^-2 | s | -2
And at the right hand side:
token | exponent
-------+----------
m | 1
s | -2
The goal of the query is to find which unit at the left hand side
matches all the tokens and exponents at the right hand side, which
would be 'm.s^-2' in the above example. The order in which the tokens
are returned can be random, there isn't really a defined order as it
doesn't change the meaning of a unit.
I do have a possible solution using array_accum [1][2] on an ordered
version (on unit,token,exponent) of these sets. It's not a pretty
solution though, I'm not happy with it - it's a transformation (from a
set to an array) where I feel none should be necessary. Isn't there a
better solution?
To illustrate, I'd prefer to perform a query somewhat like this:
SELECT unit
FROM unit, tokenize_unit('m.s^-2') AS token
WHERE each(unit.token) = each(token.token)
GROUP BY unit;
But I'm pretty sure it's not possible to use aggregates in the WHERE-
clause.
Definitions for the above are:
CREATE TYPE unit_token AS (
unit text,
exponent int
);
CREATE OR REPLACE FUNCTION tokenize_unit(unit text)
RETURNS SETOF unit_token
AS '@MODULE_PATH@', 'tokenize_unit_text'
LANGUAGE C IMMUTABLE STRICT;
CREATE TABLE token (
unit text NOT NULL REFERENCES unit,
token unit_token NOT NULL
);
[1] array_accum is an aggregate from the documentation that transforms
a set into an array.
[2] The SRF's actually return a type unit_token(token text, exponent
int) which makes using array_accum and comparisons easier.
Regards,
Alban Hertroys
--
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cut the trees and you'll see there is no forest.
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