PostgreSQL - query against GIN index of HSTORE value

Question

I have the following constructor (as a test):

CREATE TABLE product (id BIGSERIAL PRIMARY KEY, ext hstore);
CREATE INDEX ix_product_ext ON product USING GIN(ext);

INSERT
INTO    product (id, ext)
SELECT  id, ('size=>' || CEILING(10 + RANDOM() * 90) || ',mass=>' || CEILING(10 + RANDOM() * 90))::hstore
FROM    generate_series(1, 100000) id;

I have the following query, which works ok:

SELECT  COUNT(id)
FROM    (
    SELECT  id
    FROM    product
    WHERE  (ext->'size')::INT >= 41
    AND    (ext->'mass')::INT <= 20
) T

But I believe the correct way to do this is using the @> operator. I have the following, but it gives a syntax error:

SELECT  COUNT(id)
FROM    (
    SELECT  id
    FROM    product
    WHERE  ext @> 'size>=41,mass<=20'
) T

How should I write this?

Answer 1

Reading hstore documentation your (last query) size>=41 does not mean "when size is greater or equal than 41":

text => text    make single-pair hstore

Following that you can't write mass<=20, because there is no such operation. Using @> operator:

hstore @> hstore    does left operand contain right?

you can write:

SELECT count(id)
FROM product
WHERE ext @> 'size=>41,mass=>20';

However it takes only these products where size is equal to 41 and mass is equal to 20.

Answer 2

Your initial attempt is correct but you need to use (partial) btree indexes and bitmap index scans to rely on it:

create index on product(((ext->'size')::int)) where ((ext->'size') is not null);

The same for mass, and if the planner doesn't get it on the spot add two where clauses, ie where ext->'size' is not null and the same for mass.

If there is a pattern of some kind (which is likely, since most products with a size also have a mass), potentially create a multicolumn index combining the two - one sac, the other desc.

The gin index as you wrote it, along with the accompanying query (with a syntax error) will basically do the same thing but unordered; it'll be slower.