Seqscan on big table, when an Index-Usage should be possible - Mailing list pgsql-performance
From | Weinzierl Stefan |
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Subject | Seqscan on big table, when an Index-Usage should be possible |
Date | |
Msg-id | 5390C6A8.80404@Weinzierl-Stefan.de Whole thread Raw |
Responses |
Re: Seqscan on big table, when an Index-Usage should be
possible
|
List | pgsql-performance |
Hello, I'm currently testing some queries on data which I had imported from an other database-system into Postgres 9.4. After the import I did create the indexes, run an analyze and vacuum. I also played a little bit with seq_page_cost and random_page_cost. But currently I have no clue, which parameter I have to adjust, to get an query-time like the example width 'enable_seqscan=off'. Stefan > pd=> set enable_seqscan=off; > pd=> explain analyze select t.name from product p left join measurements m on p.productid=m.productid inner join measurementstypet on m.measurementstypeid=t.measurementstypeid where p.timestamp between '2013-02-01 15:00:00' and '2013-02-0521:30:00' group by t.name; > QUERY PLAN > ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------- > HashAggregate (cost=200380892.01..200380936.43 rows=4442 width=16) (actual time=34428.335..34428.693 rows=656 loops=1) > Group Key: t.name > -> Hash Join (cost=8995.44..200361772.19 rows=7647926 width=16) (actual time=103.670..30153.958 rows=5404751 loops=1) > Hash Cond: (m.measurementstypeid = t.measurementstypeid) > -> Nested Loop (cost=8279.61..200188978.03 rows=7647926 width=4) (actual time=75.939..22488.725 rows=5404751loops=1) > -> Bitmap Heap Scan on product p (cost=8279.03..662659.76 rows=526094 width=8) (actual time=75.903..326.850rows=368494 loops=1) > Recheck Cond: (("timestamp" >= '2013-02-01 15:00:00'::timestamp without time zone) AND ("timestamp"<= '2013-02-05 21:30:00'::timestamp without time zo > Heap Blocks: exact=3192 > -> Bitmap Index Scan on product_timestamp (cost=0.00..8147.51 rows=526094 width=0) (actual time=75.050..75.050rows=368494 loops=1) > Index Cond: (("timestamp" >= '2013-02-01 15:00:00'::timestamp without time zone) AND ("timestamp"<= '2013-02-05 21:30:00'::timestamp without tim > -> Index Scan using measurements_productid on measurements m (cost=0.58..347.12 rows=3214 width=12) (actualtime=0.018..0.045 rows=15 loops=368494) > Index Cond: (productid = p.productid) > -> Hash (cost=508.91..508.91 rows=16554 width=20) (actual time=27.704..27.704 rows=16554 loops=1) > Buckets: 2048 Batches: 1 Memory Usage: 686kB > -> Index Scan using measurementstype_pkey on measurementstype t (cost=0.29..508.91 rows=16554 width=20)(actual time=0.017..15.719 rows=16554 loops=1) > Planning time: 2.176 ms > Execution time: 34429.080 ms > (17 Zeilen) > > > Zeit: 34432,187 ms > pd=> set enable_seqscan=on; > SET > Zeit: 0,193 ms > pd=> explain analyze select t.name from product p left join measurements m on p.productid=m.productid inner join measurementstypet on m.measurementstypeid=t.measurementstypeid where p.timestamp between '2013-02-01 15:00:00' and '2013-02-0521:30:00' group by t.name; > QUERY PLAN > ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------- > HashAggregate (cost=108645282.49..108645326.91 rows=4442 width=16) (actual time=5145182.269..5145182.656 rows=656 loops=1) > Group Key: t.name > -> Hash Join (cost=671835.40..108626162.68 rows=7647926 width=16) (actual time=2087822.232..5141351.539 rows=5404751loops=1) > Hash Cond: (m.measurementstypeid = t.measurementstypeid) > -> Hash Join (cost=671291.94..108453540.88 rows=7647926 width=4) (actual time=2087800.816..5134312.822 rows=5404751loops=1) > Hash Cond: (m.productid = p.productid) > -> Seq Scan on measurements m (cost=0.00..49325940.08 rows=2742148608 width=12) (actual time=0.007..2704591.045rows=2742146806 loops=1) > -> Hash (cost=662659.76..662659.76 rows=526094 width=8) (actual time=552.480..552.480 rows=368494 loops=1) > Buckets: 16384 Batches: 4 Memory Usage: 2528kB > -> Bitmap Heap Scan on product p (cost=8279.03..662659.76 rows=526094 width=8) (actual time=73.353..302.482rows=368494 loops=1) > Recheck Cond: (("timestamp" >= '2013-02-01 15:00:00'::timestamp without time zone) AND ("timestamp"<= '2013-02-05 21:30:00'::timestamp without t > Heap Blocks: exact=3192 > -> Bitmap Index Scan on product_timestamp (cost=0.00..8147.51 rows=526094 width=0) (actualtime=72.490..72.490 rows=368494 loops=1) > Index Cond: (("timestamp" >= '2013-02-01 15:00:00'::timestamp without time zone) AND ("timestamp"<= '2013-02-05 21:30:00'::timestamp witho > -> Hash (cost=336.54..336.54 rows=16554 width=20) (actual time=21.377..21.377 rows=16554 loops=1) > Buckets: 2048 Batches: 1 Memory Usage: 686kB > -> Seq Scan on measurementstype t (cost=0.00..336.54 rows=16554 width=20) (actual time=0.008..9.849 rows=16554loops=1) > Planning time: 2.236 ms > Execution time: 5145183.471 ms > (19 Zeilen) > > Zeit: 5145186,786 ms > pd=> \d measurements > Tabelle „public.measurements“ > Spalte | Typ | Attribute > --------------------+-----------------------------+--------------------------------------------------------------------------- > measurementsid | bigint | not null Vorgabewert nextval('measurements_measurementsid_seq'::regclass) > value | text | not null > lowerlimit | text | > upperlimit | text | > measurementstypeid | integer | not null > productid | bigint | not null > timestamp | timestamp without time zone | > state | character varying(20) | not null Vorgabewert 'Unknown'::character varying > Indexe: > "measurements_pkey" PRIMARY KEY, btree (measurementsid) > "measurements_measurementstypeid" btree (measurementstypeid) > "measurements_productid" btree (productid) > > pd=> \d product > Tabelle „public.product“ > Spalte | Typ | Attribute > ------------------+-----------------------------+----------------------------------------------------------------- > productid | bigint | not null Vorgabewert nextval('product_productid_seq'::regclass) > ordermaterialsid | integer | not null > testerid | integer | > equipmentid | integer | not null > timestamp | timestamp without time zone | not null > state | character varying(20) | not null Vorgabewert 'Unknown'::character varying > exported | character varying(1) | not null Vorgabewert 'N'::character varying > mc_selectionid | integer | > Indexe: > "product_pkey" PRIMARY KEY, btree (productid) > "product_equipmentid" btree (equipmentid) > "product_exported" btree (exported) > "product_mc_selectionid" btree (mc_selectionid) > "product_ordermaterialsid" btree (ordermaterialsid) > "product_state" btree (state) > "product_testerid" btree (testerid) > "product_timestamp" btree ("timestamp") > Fremdschlüssel-Constraints: > "fk_equipmentid" FOREIGN KEY (equipmentid) REFERENCES equipment(equipmentid) ON UPDATE CASCADE ON DELETE RESTRICT > "fk_mc_selectionid" FOREIGN KEY (mc_selectionid) REFERENCES mc_selection(mc_selectionid) ON UPDATE CASCADE ON DELETESET NULL > "fk_ordermaterialsid" FOREIGN KEY (ordermaterialsid) REFERENCES ordermaterials(ordermaterialsid) ON UPDATE CASCADEON DELETE RESTRICT > "fk_testerid" FOREIGN KEY (testerid) REFERENCES tester(testerid) ON UPDATE CASCADE ON DELETE RESTRICT > > pd=> \d measurementstype > Tabelle „public.measurementstype“ > Spalte | Typ | Attribute > --------------------+------------------------+----------------------------------------------------------------------------------- > measurementstypeid | integer | not null Vorgabewert nextval('measurementstype_measurementstypeid_seq'::regclass) > datatype | character varying(20) | not null Vorgabewert 'char'::character varying > name | character varying(255) | not null > description | character varying(255) | Vorgabewert NULL::character varying > unit | character varying(20) | Vorgabewert NULL::character varying > step | integer | > stepdescription | character varying(255) | Vorgabewert NULL::character varying > permissionlevel | integer | not null Vorgabewert 0 > Indexe: > "measurementstype_pkey" PRIMARY KEY, btree (measurementstypeid) > "measurementstype_datatype" btree (datatype) > "measurementstype_name" btree (name) > "measurementstype_step" btree (step) > "measurementstype_stepdescription" btree (stepdescription) >
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