Hi,
Is it intended that indexes are not pushed down to union all
subqueries if even a single select contains a where clause? Is this
just not implemented, is it impossible to implement or am I doing
something wrong?
The following query does a SeqScan for "bikes" and "cars" tables even
though IndexScan on their column "dealer_name" would be a magnitude
faster. (Schema with sample data at the bottom of this mail)
WITH targets as ( select 'bike' vehicle, id, dealer_name FROM bikes WHERE frame_size = 52 union all select 'car' vehicle, id, dealer_name FROM cars -- In the real use case I have here dozens of tables )
SELECT dealers.name dealer, targets.vehicle, targets.id
FROM dealers JOIN targets ON dealers.name = targets.dealer_name WHERE dealers.id in (54,12,456,315,468)
If the WHERE clause from the "bikes" subquery is removed then first
Index Scan on dealers_pk is made and then Bitmap Index Scans on
"bikes" and "cars" table using indexes on "dealer_name" columns.
--------------------------------------------------------------- -- Available also at: https://www.db-fiddle.com/f/cEXt8HXSaQzsQ2yBDA2Z4H/7
CREATE TABLE dealers AS
SELECT id, (SELECT string_agg(CHR(65+(random() * 25)::integer), '') FROM
generate_series(1, 4) WHERE id>0) name
FROM generate_series(1, 1000) AS id
;
ALTER TABLE dealers ADD primary key (id);
CREATE INDEX ON dealers(name);
CREATE TABLE bikes AS
SELECT generate_series AS id, (SELECT name FROM dealers WHERE dealers.id = (SELECT
(random()*1000)::int WHERE generate_series>0)) AS dealer_name, (random()*12+50)::int as frame_size
FROM generate_series(1, 100000);
ALTER TABLE bikes ADD primary key (id);
CREATE INDEX ON bikes(dealer_name);
CREATE TABLE cars AS
SELECT generate_series as id, (SELECT name FROM dealers WHERE dealers.id = (SELECT
(random()*1000)::int WHERE generate_series>0)) AS dealer_name, (random()*7+14)::int as wheel_size
FROM generate_series(1, 100000);
ALTER TABLE cars ADD primary key (id);
CREATE INDEX ON cars(dealer_name);
ANALYZE;
--
- Lauri
With an INNER JOIN, both tables must be fully checked/matched (check using EXPLAIN ANALYSIS -> HashJoin), so the index cannot be used here.
Sorry, didn't consider the WITH part.Please share the detailed query plan for more info.
The "bikes" subquery uses field "frame_size" in WHERE clause but the field does not have an index...
ADD: Consider whether it might make sense to take a more generalist approach by only having one entity vehicle with the distinction "car", "bike", etc...? ADD: Consider to do more complex "detailed" SELECTs that are unioned (if that is really needed)?
