Re: Combination of partial and full indexes - Mailing list pgsql-performance

I thought that first column from left in multi-column index can and will be used just as it would be a single column
index.

It doesn’t seem to work with unqiue indexes, which ultimetly makes sense.

Thank you Gerardo.

> On 07 Jun 2016, at 19:36, Gerardo Herzig <gherzig@fmed.uba.ar> wrote:
>
> I dont think offers_source_id_o_key_idx will be used at all. It is a UNIQUE index on (source_id, o_key), but your
querydoes not filter for any "o_key", so reading that index does not provide the pointers needed to fetch the actual
datain the table. 
>
> I will try an index on source_id, offer_next_update(offers.update_ts, offers.update_freq) and see what happens
>
> HTH
> Gerardo
>
> ----- Mensaje original -----
>> De: "Rafał Gutkowski" <goodkowski@gmail.com>
>> Para: pgsql-performance@postgresql.org
>> Enviados: Martes, 7 de Junio 2016 10:39:14
>> Asunto: [PERFORM] Combination of partial and full indexes
>>
>>
>> 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
>> $function$
>>
>>
>>
>>