Hi. I had a fight with a query planner because it doesn’t listen. There are two indexes: - with _expression_ in descending order: "offers_offer_next_update_idx" btree (offer_next_update(update_ts, update_freq) DESC) WHERE o_archived = false - unique with two columns: "offers_source_id_o_key_idx" UNIQUE, btree (source_id, o_key) Here's the query with filter for offers.source_id columns which is pretty slow because "offers_source_id_o_key_idx" is not used: EXPLAIN ANALYZE SELECT offers.o_url AS offers_o_url FROM offers WHERE offers.source_id = 1 AND offers.o_archived = false AND now() > offer_next_update(offers.update_ts, offers.update_freq) ORDER BY offer_next_update(offers.update_ts, offers.update_freq) DESC LIMIT 1000; Limit (cost=0.68..23403.77 rows=1000 width=116) (actual time=143.544..147.870 rows=1000 loops=1) -> Index Scan using offers_offer_next_update_idx on offers (cost=0.68..1017824.69 rows=43491 width=116) (actual time=143.542..147.615 rows=1000 loops=1) Index Cond: (now() > offer_next_update(update_ts, update_freq)) Filter: (source_id = 1) Rows Removed by Filter: 121376 Total runtime: 148.023 ms When I remove filter on offers.source_id, query plan looks like this: EXPLAIN ANALYZE SELECT offers.o_url AS offers_o_url FROM offers WHERE offers.o_archived = false AND now() > offer_next_update(offers.update_ts, offers.update_freq) ORDER BY offer_next_update(offers.update_ts, offers.update_freq) DESC LIMIT 1000; Limit (cost=0.68..4238.27 rows=1000 width=116) (actual time=0.060..3.877 rows=1000 loops=1) -> Index Scan using offers_offer_next_update_idx on offers (cost=0.68..1069411.78 rows=252363 width=116) (actual time=0.058..3.577 rows=1000 loops=1) Index Cond: (now() > offer_next_update(update_ts, update_freq)) Total runtime: 4.031 ms I even tried to change orders of conditions in second query but it doesn't seem to make a difference for a planner. Shouldn't query planner use offers_source_id_o_key_idx to speed up query above? PostgreSQL version: PostgreSQL 9.3.12 on x86_64-unknown-linux-gnu, compiled by gcc (Ubuntu 4.8.4-2ubuntu1~14.04.1) 4.8.4, 64-bit Configuration: name | current_setting | source ------------------------------+----------------------------------------+---------------------- application_name | psql | client checkpoint_completion_target | 0.9 | configuration file checkpoint_segments | 3 | configuration file client_encoding | UTF8 | client DateStyle | ISO, MDY | configuration file default_text_search_config | pg_catalog.english | configuration file effective_cache_size | 128MB | configuration file external_pid_file | /var/run/postgresql/9.3-main.pid | configuration file lc_messages | en_US.UTF-8 | configuration file lc_monetary | en_US.UTF-8 | configuration file lc_numeric | en_US.UTF-8 | configuration file lc_time | en_US.UTF-8 | configuration file max_connections | 100 | configuration file max_locks_per_transaction | 168 | configuration file max_stack_depth | 2MB | environment variable port | 5432 | configuration file shared_buffers | 4GB | configuration file temp_buffers | 12MB | configuration file unix_socket_directories | /var/run/postgresql | configuration file work_mem | 16MB | configuration file Definitions: CREATE OR REPLACE FUNCTION public.offer_next_update(last timestamp without time zone, minutes smallint) RETURNS timestamp without time zone LANGUAGE plpgsql IMMUTABLE AS $function$ BEGIN RETURN last + (minutes || ' min')::interval; END $function$ |
