Hi, PostgreSQL version : PostgreSQL 9.6.2 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 4.8.3 20140911 (Red Hat 4.8.3-9), 64-bit
We have noticed huge difference interms of execution plan ( response time) , When we pass the direct values Vs inner query to IN clause. High level details of the use case are as follows - As part of the SQL there are 2 tables named Process_instance (master) and Process_activity ( child) - Wanted to fetch TOP 50 rows from Process_activity table for the given values of the Process_instance. - When we used Inner Join / Inner query ( query1) between parent table and child table , LIMIT is not really taking in to account. Instead it is fetching more rows and columns that required, and finally limiting the result - *Query1* web_1=> EXPLAIN (ANALYZE, COSTS, VERBOSE, BUFFERS) SELECT pa.process_activity_id FROM process_activity pa WHERE pa.app_id = '427380312000560' AND pa.created > '1970-01-01 00:00:00' AND pa.process_instance_id in *(SELECT pi.process_instance_id FROM process_instance pi WHERE pi.user_id = '317079413683604' AND pi.app_id = '427380312000560')* ORDER BY pa.process_instance_id,pa.created limit 50; QUERY PLAN ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ Limit (cost=1071.47..1071.55 rows=31 width=24) (actual time=85.958..85.991 rows=50 loops=1) Output: pa.process_activity_id, pa.process_instance_id, pa.created Buffers: shared hit=43065 -> Sort (cost=1071.47..1071.55 rows=31 width=24) (actual time=85.956..85.971 rows=50 loops=1) Output: pa.process_activity_id, pa.process_instance_id, pa.created Sort Key: pa.process_instance_id, pa.created Sort Method: top-N heapsort Memory: 28kB Buffers: shared hit=43065 -> Nested Loop (cost=1.14..1070.70 rows=31 width=24) (actual time=0.031..72.183 rows=46992 loops=1) Output: pa.process_activity_id, pa.process_instance_id, pa.created Buffers: shared hit=43065 -> Index Scan using fki_conv_konotor_user_user_id on public.process_instance pi (cost=0.43..2.66 rows=1 width=8) (actual time=0.010..0.013 rows=2 loops=1) Output: pi.process_instance_id Index Cond: (pi.user_id = '317079413683604'::bigint) Filter: (pi.app_id = '427380312000560'::bigint) Buffers: shared hit=5 -> Index Scan using process_activity_process_instance_id_app_id_created_idx on public.process_activity pa (cost=0.70..1053.80 rows=1425 width=24) (actual time=0.015..20.702 rows=*23496* loops=2) * Output: pa.process_activity_id, pa.process_activity_type, pa.voice_url, pa.process_activity_user_id, pa.app_id, pa.process_instance_id, pa.alias, pa.read_by_user, pa.source, pa.label_category_id, pa.label_id, pa.csat_response_id, pa.process_activity_fragments, pa.created, pa.updated, pa.rule_id, pa.marketing_reply_id, pa.delivered_at, pa.reply_fragments, pa.status_fragment, pa.internal_meta, pa.interaction_id, pa.do_not_translate, pa.should_translate, pa.in_reply_to* Index Cond: ((pa.process_instance_id = pi.process_instance_id) AND (pa.app_id = '427380312000560'::bigint) AND (pa.created > '1970-01-01 00:00:00'::timestamp without time zone)) Buffers: shared hit=43060 Planning time: 0.499 ms Execution time: 86.040 ms (22 rows) *Query 2* web_1=> EXPLAIN (ANALYZE, COSTS, VERBOSE, BUFFERS) SELECT pa.process_activity_id AS m_process_activity_id FROM process_activity m WHERE pa.app_id = '427380312000560' AND pa.created > '1970-01-01 00:00:00' AND pa.process_instance_id in ( *240117466018927,325820556706970,433008275197305*) ORDER BY pa.process_instance_id,pa.created limit 50; QUERY PLAN --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Limit (cost=0.70..37.66 rows=50 width=24) (actual time=0.023..0.094 rows=50 loops=1) Output: process_activity_id, process_instance_id, created Buffers: shared hit=50 -> Index Scan using process_activity_process_instance_id_app_id_created_idx on public.process_activity pa (cost=0.70..3124.97 rows=4226 width=24) (actual time=0.022..0.079 *rows=50* loops=1) Output: process_activity_id, process_instance_id, created Index Cond: ((pa.process_instance_id = ANY ('{140117466018927,225820556706970,233008275197305}'::bigint[])) AND (pa.app_id = '427380312000560'::bigint) AND (pa.created > '1970-01-01 00:00:00'::timestamp without time zone)) Buffers: shared hit=50 Planning time: 0.167 ms Execution time: 0.137 ms (9 rows) Can someone explain - Why It is fetching more columns and more rows, incase of inner query ? - Is there any option to really limit values with INNER JOIN, INNER query ? If yes, can you please share information on this ? Thanks in advance for your time and suggestions. Regards, Amar