Re: [PERFORM] slow join not using index properly
Hi Stefan, stupid me - Ive missed some with RECURSIVE qq(cont_key, anc_key) AS ( SELECT a1.context_key, ancestor_key FROM virtual_ancestors a1 UNION select ( -- here, in the union SELECT a1.context_key, a1.ancestor_key FROM virtual_ancestors a1 WHERE a1.context_key cont_key ORDER BY a1.context_key LIMIT 1 ) from qq where cont_key is not null -- and here ) SELECT distinct a.cont_key FROM qq a, collection_data, virtual_ancestors a2 WHERE a.cont_key IS NOT NULL AND a.anc_key = collection_data.context_key AND collection_data.collection_ context_key = a2.context_key AND a2.ancestor_key = 1072173; sorry for disorientating On Mon, Mar 24, 2014 at 7:40 PM, Stefan Amshey srams...@gmail.com wrote: Hi Ilya- Thanks so much for taking a stab at optimizing that query. I had to fiddle a bit with your proposed version in order to get it function. Here's what I came up with in the end: with RECURSIVE qq(cont_key, anc_key) AS ( SELECT a1.context_key, ancestor_key FROM virtual_ancestors a1 UNION ( SELECT a1.context_key, a1.ancestor_key FROM virtual_ancestors a1, qq WHERE context_key qq.cont_key ORDER BY context_key LIMIT 1 ) ) SELECT distinct a.cont_key FROM qq a, collection_data, virtual_ancestors a2 WHERE a.cont_key IS NOT NULL AND a.anc_key = collection_data.context_key AND collection_data.collection_context_key = a2.context_key AND a2.ancestor_key = 1072173; I had to drop the MIN( a1.context_key ) and LIMIT 1 from your version off of the first select statement in order to avoid syntax issues or other errors. The version above does produce the same counts as the original, but in the end it wasn't really a win for us. Here's the plan it produced: HashAggregate (cost=707724.36..707726.36 rows=200 width=4) (actual time=27638.844..27639.706 rows=3522 loops=1) Buffers: shared hit=79323, temp read=49378 written=47557 CTE qq - Recursive Union (cost=0.00..398869.78 rows=10814203 width=8) (actual time=0.018..20196.397 rows=10821685 loops=1) Buffers: shared hit=74449, temp read=49378 written=23779 - Seq Scan on virtual_ancestors a1 (cost=0.00..182584.93 rows=10814193 width=8) (actual time=0.010..2585.411 rows=10821685 loops=1) Buffers: shared hit=74443 - Limit (cost=0.00..0.08 rows=1 width=8) (actual time=7973.297..7973.298 rows=1 loops=1) Buffers: shared hit=6, temp read=49378 written=1 - Nested Loop (cost=0.00..30881281719119.79 rows=389822567470830 width=8) (actual time=7973.296..7973.296 rows=1 loops=1) Join Filter: (a1.context_key qq.cont_key) Rows Removed by Join Filter: 22470607 Buffers: shared hit=6, temp read=49378 written=1 - Index Scan using virtual_context_key_idx on virtual_ancestors a1 (cost=0.00..18206859.46 rows=10814193 width=8) (actual time=0.018..0.036 rows=3 loops=1) Buffers: shared hit=6 - WorkTable Scan on qq (cost=0.00..2162838.60 rows=108141930 width=4) (actual time=0.008..1375.445 rows=7490203 loops=3) Buffers: temp read=49378 written=1 - Hash Join (cost=25283.37..308847.31 rows=2905 width=4) (actual time=449.167..27629.759 rows=13152 loops=1) Hash Cond: (a.anc_key = collection_data.context_key) Buffers: shared hit=79323, temp read=49378 written=47557 - CTE Scan on qq a (cost=0.00..216284.06 rows=10760132 width=8) (actual time=0.021..25265.179 rows=10821685 loops=1) Filter: (cont_key IS NOT NULL) Buffers: shared hit=74449, temp read=49378 written=47557 - Hash (cost=25282.14..25282.14 rows=98 width=4) (actual time=373.836..373.836 rows=2109 loops=1) Buckets: 1024 Batches: 1 Memory Usage: 75kB Buffers: shared hit=4874 - Hash Join (cost=17557.15..25282.14 rows=98 width=4) (actual time=368.374..373.013 rows=2109 loops=1) Hash Cond: (a2.context_key = collection_data.collection_context_key) Buffers: shared hit=4874 - Index Only Scan using virtual_ancestors_pkey on virtual_ancestors a2 (cost=0.00..238.57 rows=272 width=4) (actual time=0.029..1.989 rows=1976 loops=1) Index Cond: (ancestor_key = 1072173) Heap Fetches: 917 Buffers: shared hit=883 - Hash (cost=10020.40..10020.40 rows=602940 width=8) (actual time=368.057..368.057 rows=603066 loops=1) Buckets: 65536 Batches: 1 Memory Usage:
Re: [PERFORM] slow join not using index properly
Hi Ilya- Thanks so much for taking a stab at optimizing that query. I had to fiddle a bit with your proposed version in order to get it function. Here's what I came up with in the end: with RECURSIVE qq(cont_key, anc_key) AS ( SELECT a1.context_key, ancestor_key FROM virtual_ancestors a1 UNION ( SELECT a1.context_key, a1.ancestor_key FROM virtual_ancestors a1, qq WHERE context_key qq.cont_key ORDER BY context_key LIMIT 1 ) ) SELECT distinct a.cont_key FROM qq a, collection_data, virtual_ancestors a2 WHERE a.cont_key IS NOT NULL AND a.anc_key = collection_data.context_key AND collection_data.collection_context_key = a2.context_key AND a2.ancestor_key = 1072173; I had to drop the MIN( a1.context_key ) and LIMIT 1 from your version off of the first select statement in order to avoid syntax issues or other errors. The version above does produce the same counts as the original, but in the end it wasn't really a win for us. Here's the plan it produced: HashAggregate (cost=707724.36..707726.36 rows=200 width=4) (actual time=27638.844..27639.706 rows=3522 loops=1) Buffers: shared hit=79323, temp read=49378 written=47557 CTE qq - Recursive Union (cost=0.00..398869.78 rows=10814203 width=8) (actual time=0.018..20196.397 rows=10821685 loops=1) Buffers: shared hit=74449, temp read=49378 written=23779 - Seq Scan on virtual_ancestors a1 (cost=0.00..182584.93 rows=10814193 width=8) (actual time=0.010..2585.411 rows=10821685 loops=1) Buffers: shared hit=74443 - Limit (cost=0.00..0.08 rows=1 width=8) (actual time=7973.297..7973.298 rows=1 loops=1) Buffers: shared hit=6, temp read=49378 written=1 - Nested Loop (cost=0.00..30881281719119.79 rows=389822567470830 width=8) (actual time=7973.296..7973.296 rows=1 loops=1) Join Filter: (a1.context_key qq.cont_key) Rows Removed by Join Filter: 22470607 Buffers: shared hit=6, temp read=49378 written=1 - Index Scan using virtual_context_key_idx on virtual_ancestors a1 (cost=0.00..18206859.46 rows=10814193 width=8) (actual time=0.018..0.036 rows=3 loops=1) Buffers: shared hit=6 - WorkTable Scan on qq (cost=0.00..2162838.60 rows=108141930 width=4) (actual time=0.008..1375.445 rows=7490203 loops=3) Buffers: temp read=49378 written=1 - Hash Join (cost=25283.37..308847.31 rows=2905 width=4) (actual time=449.167..27629.759 rows=13152 loops=1) Hash Cond: (a.anc_key = collection_data.context_key) Buffers: shared hit=79323, temp read=49378 written=47557 - CTE Scan on qq a (cost=0.00..216284.06 rows=10760132 width=8) (actual time=0.021..25265.179 rows=10821685 loops=1) Filter: (cont_key IS NOT NULL) Buffers: shared hit=74449, temp read=49378 written=47557 - Hash (cost=25282.14..25282.14 rows=98 width=4) (actual time=373.836..373.836 rows=2109 loops=1) Buckets: 1024 Batches: 1 Memory Usage: 75kB Buffers: shared hit=4874 - Hash Join (cost=17557.15..25282.14 rows=98 width=4) (actual time=368.374..373.013 rows=2109 loops=1) Hash Cond: (a2.context_key = collection_data.collection_context_key) Buffers: shared hit=4874 - Index Only Scan using virtual_ancestors_pkey on virtual_ancestors a2 (cost=0.00..238.57 rows=272 width=4) (actual time=0.029..1.989 rows=1976 loops=1) Index Cond: (ancestor_key = 1072173) Heap Fetches: 917 Buffers: shared hit=883 - Hash (cost=10020.40..10020.40 rows=602940 width=8) (actual time=368.057..368.057 rows=603066 loops=1) Buckets: 65536 Batches: 1 Memory Usage: 23558kB Buffers: shared hit=3991 - Seq Scan on collection_data (cost=0.00..10020.40 rows=602940 width=8) (actual time=0.006..146.447 rows=603066 loops=1) Buffers: shared hit=3991 Total runtime: 27854.200 ms I also tried including the MIN( a1.context_key ) in the first select statement as you had written it, but upon doing that it became necessary to add a GROUP BY clause, and doing that changed the final number of rows selected: ERROR: column a1.ancestor_key must appear in the GROUP BY clause or be used in an aggregate function LINE 4: min( a1.context_key ), ancestor_key ^ Including the LIMIT 1 at the end of the first select statement gave a syntax error that I couldn't seem to
Re: [PERFORM] slow join not using index properly
Hi Stefan! Probably you need to rewrite your query like this (check it first): with RECURSIVE qq(cont_key, anc_key) as ( select min(a1.context_key), ancestor_key from virtual_ancestors a1 union select (SELECT a1.context_key, ancestor_key FROM virtual_ancestors a1 where context_key cont_key order by context_key limit 1) from qq where cont_key is not null ) select a1.cont_key from qq a1, collection_data, virtual_ancestors a2 WHERE a1.anc_key = collection_data.context_key AND collection_data.collection_context_key = a2.context_key AND a2.ancestor_key = ? best regards, Ilya On Fri, Mar 21, 2014 at 12:56 AM, Stefan Amshey srams...@gmail.com wrote: We have a slow performing query that we are trying to improve, and it appears to be performing a sequential scan at a point where it should be utilizing an index. Can anyone tell me why postgres is opting to do it this way? The original query is as follows: SELECT DISTINCT a1.context_key FROM virtual_ancestors a1, collection_data, virtual_ancestors a2 WHERE a1.ancestor_key = collection_data.context_key AND collection_data.collection_context_key = a2.context_key AND a2.ancestor_key = ? The key relationships should all using indexed columns, but the query plan that postgres comes up with ends up performing a sequential scan on the collection_data table (in this case about 602k rows) where we would have expected it to utilize the index: HashAggregate (cost=60905.73..60935.73 rows=3000 width=4) (actual time=3366.165..3367.354 rows=3492 loops=1) Buffers: shared hit=16291 read=1222 - Nested Loop (cost=17546.26..60898.23 rows=3000 width=4) (actual time=438.332..3357.918 rows=13037 loops=1) Buffers: shared hit=16291 read=1222 - Hash Join (cost=17546.26..25100.94 rows=98 width=4) (actual time=408.554..415.767 rows=2092 loops=1) Hash Cond: (a2.context_key = collection_data.collection_context_key) Buffers: shared hit=4850 read=3 - Index Only Scan using virtual_ancestors_pkey on virtual_ancestors a2 (cost=0.00..233.32 rows=270 width=4) (actual time=8.532..10.703 rows=1960 loops=1) Index Cond: (ancestor_key = 1072173) Heap Fetches: 896 Buffers: shared hit=859 read=3 - Hash (cost=10015.56..10015.56 rows=602456 width=8) (actual time=399.708..399.708 rows=602570 loops=1) Buckets: 65536 Batches: 1 Memory Usage: 23538kB Buffers: shared hit=3991 sequential scan occurs here ## - Seq Scan on collection_data (cost=0.00..10015.56 rows=602456 width=8) (actual time=0.013..163.509 rows=602570 loops=1) Buffers: shared hit=3991 - Index Only Scan using virtual_ancestors_pkey on virtual_ancestors a1 (cost=0.00..360.70 rows=458 width=8) (actual time=1.339..1.403 rows=6 loops=2092) Index Cond: (ancestor_key = collection_data.context_key) Heap Fetches: 7067 Buffers: shared hit=11441 read=1219 Total runtime: 3373.058 ms The table definitions are as follows: Table public.virtual_ancestors Column| Type | Modifiers --+--+--- ancestor_key | integer | not null context_key | integer | not null degree | smallint | not null Indexes: virtual_ancestors_pkey PRIMARY KEY, btree (ancestor_key, context_key) virtual_context_key_idx btree (context_key) Foreign-key constraints: virtual_ancestors_ancestor_key_fkey FOREIGN KEY (ancestor_key) REFERENCES contexts(context_key) virtual_ancestors_context_key_fkey FOREIGN KEY (context_key) REFERENCES contexts(context_key) Table public.collection_data Column | Type | Modifiers +--+--- collection_context_key | integer | not null context_key| integer | not null type| character varying(1) | not null source| character varying(1) | not null Indexes: collection_data_context_key_idx btree (context_key) collection_data_context_key_index btree (collection_context_key) CLUSTER Foreign-key constraints: collection_data_collection_context_key_fkey FOREIGN KEY (collection_context_key) REFERENCES contexts(context_key) ON DELETE CASCADE collection_data_context_key_fkey FOREIGN KEY (context_key) REFERENCES contexts(context_key) ON DELETE CASCADE Can anyone suggest a way that we can get postgres to use the collection_data_context_key_index properly? I thought that it might be related to the fact that collection_data_context_key_index is a CLUSTERED index, but we did some basic experimentation that seems to indicate otherwise, i.e. the bad plan
Re: [PERFORM] slow join not using index properly
On 21-03-14 00:56, Stefan Amshey wrote: We have a slow performing query that we are trying to improve, and it appears to be performing a sequential scan at a point where it should be utilizing an index. Can anyone tell me why postgres is opting to do it this way? The original query is as follows: SELECT DISTINCT a1.context_key FROM virtual_ancestors a1, collection_data, virtual_ancestors a2 WHERE a1.ancestor_key = collection_data.context_key AND collection_data.collection_context_key = a2.context_key AND a2.ancestor_key = ? The key relationships should all using indexed columns, but the query plan that postgres comes up with ends up performing a sequential scan on the collection_data table (in this case about 602k rows) where we would have expected it to utilize the index: HashAggregate (cost=60905.73..60935.73 rows=3000 width=4) (actual time=3366.165..3367.354 rows=3492 loops=1) Buffers: shared hit=16291 read=1222 - Nested Loop (cost=17546.26..60898.23 rows=3000 width=4) (actual time=438.332..3357.918 rows=13037 loops=1) Buffers: shared hit=16291 read=1222 - Hash Join (cost=17546.26..25100.94 rows=98 width=4) (actual time=408.554..415.767 rows=2092 loops=1) Hash Cond: (a2.context_key = collection_data.collection_context_key) Buffers: shared hit=4850 read=3 - Index Only Scan using virtual_ancestors_pkey on virtual_ancestors a2 (cost=0.00..233.32 rows=270 width=4) (actual time=8.532..10.703 rows=1960 loops=1) Index Cond: (ancestor_key = 1072173) Heap Fetches: 896 Buffers: shared hit=859 read=3 - Hash (cost=10015.56..10015.56 rows=602456 width=8) (actual time=399.708..399.708 rows=602570 loops=1) Buckets: 65536 Batches: 1 Memory Usage: 23538kB Buffers: shared hit=3991 sequential scan occurs here ## - Seq Scan on collection_data (cost=0.00..10015.56 rows=602456 width=8) (actual time=0.013..163.509 rows=602570 loops=1) Buffers: shared hit=3991 - Index Only Scan using virtual_ancestors_pkey on virtual_ancestors a1 (cost=0.00..360.70 rows=458 width=8) (actual time=1.339..1.403 rows=6 loops=2092) Index Cond: (ancestor_key = collection_data.context_key) Heap Fetches: 7067 Buffers: shared hit=11441 read=1219 Total runtime: 3373.058 ms The table definitions are as follows: Table public.virtual_ancestors Column| Type | Modifiers --+--+--- ancestor_key | integer | not null context_key | integer | not null degree | smallint | not null Indexes: virtual_ancestors_pkey PRIMARY KEY, btree (ancestor_key, context_key) virtual_context_key_idx btree (context_key) Foreign-key constraints: virtual_ancestors_ancestor_key_fkey FOREIGN KEY (ancestor_key) REFERENCES contexts(context_key) virtual_ancestors_context_key_fkey FOREIGN KEY (context_key) REFERENCES contexts(context_key) Table public.collection_data Column | Type | Modifiers +--+--- collection_context_key | integer | not null context_key| integer | not null type| character varying(1) | not null source| character varying(1) | not null Indexes: collection_data_context_key_idx btree (context_key) collection_data_context_key_index btree (collection_context_key) CLUSTER Foreign-key constraints: collection_data_collection_context_key_fkey FOREIGN KEY (collection_context_key) REFERENCES contexts(context_key) ON DELETE CASCADE collection_data_context_key_fkey FOREIGN KEY (context_key) REFERENCES contexts(context_key) ON DELETE CASCADE Can anyone suggest a way that we can get postgres to use the collection_data_context_key_index properly? I thought that it might be related to the fact that collection_data_context_key_index is a CLUSTERED index, but we did some basic experimentation that seems to indicate otherwise, i.e. the bad plan persists despite re-clustering the index. We are using PostgreSQL 9.2.5 on x86_64-unknown-linux-gnu, compiled by gcc (Debian 4.4.5-8) 4.4.5, 64-bit Interestingly, on an instance running PostgreSQL 9.2.4 on x86_64-unknown-linux-gnu, compiled by gcc (Debian 4.4.5-8) 4.4.5, 64-bit where I copied the 2 tables over to a temporary database, the plan comes out differently: HashAggregate (cost=39692.03..39739.98 rows=4795 width=4) (actual time=73.285..75.141 rows=3486 loops=1) Buffers: shared hit=22458 - Nested Loop (cost=0.00..39680.05 rows=4795 width=4) (actual time=0.077..63.116 rows=13007 loops=1) Buffers: shared hit=22458 - Nested Loop (cost=0.00..32823.38 rows=164 width=4) (actual time=0.056..17.685 rows=2084 loops=1) Buffers: shared hit=7529 - Index Only
