Thank you Pavel.
The cost of unpacking hstore comparing to non-hstore could be calculated by:
Seq scan on hstore table + hash join with seg1 table:
Hstore: 416.741+ 34619.879 =~34 seconds
Non-hstore: 8858.594 +26477.652 =~ 34 seconds
The subsequent hash-aggregate and sort operation should be working on the unpacked hstore rows which has same row counts as non-hstore table. however, timing on those operations actually makes the big difference.
I don’t quite get why…
Thanks,
Suya
From: Pavel Stehule [mailto:pavel.stehule@xxxxxxxxx]
Sent: Monday, September 01, 2014 4:22 PM
To: Huang, Suya
Cc: pgsql-performance@xxxxxxxxxxxxxx
Subject: Re: query performance with hstore vs. non-hstore
Hi
In this use case hstore should not help .. there is relative high overhead related with unpacking hstore -- so classic schema is better.
Hstore should not to replace well normalized schema - it should be a replace for some semi normalized structures as EAV.
Hstore can have some profit from TOAST .. comprimation, less system data overhead, but this advantage started from some length of data. You should to see this benefit on table size. When table with HStore is less than without, then there is benefit of Hstore. Last benefit of Hstore are indexes over tuple (key, value) .. but you don't use it.
Regards
Pavel
2014-09-01 8:10 GMT+02:00 Huang, Suya <Suya.Huang@xxxxxxxxxxxxxxx>:
Hi ,
I’m tweaking table layout to get better performance of query. One table doesn’t use hstore but expand all metrics of cha_type to different rows. The other table has hstore for metrics column as cha_type->metrics so it has less records than the first one.
I would be expecting the query on seconds table has better performance than the first one. However, it’s not the case at all. I’m wondering if there’s something wrong with my execution plan? With the hstore table, the optimizer has totally wrong estimation on row counts at hash aggregate stage and it takes 34 seconds on hash-join,25 seconds on hash-aggregate, 10 seconds on sort. However, with non-hstore table, it takes 17 seconds on hash join, 18 seconds on hashaggregate and 2 seconds on sort.
Can someone help me to explain why this is happening? And is there a way to fine-tune the query?
Table structure
dev=# \d+ weekly_non_hstore
Table "test.weekly_non_hstore"
Column | Type | Modifiers | Storage | Stats target | Description
----------+------------------------+-----------+----------+--------------+-------------
date | date | | plain | |
ref_id | character varying(256) | | extended | |
cha_typel | text | | extended | |
visits | double precision | | plain | |
pages | double precision | | plain | |
duration | double precision | | plain | |
Has OIDs: no
Tablespace: "tbs_data"
dev=# \d+ weekly_hstore
Table "test.weekly_hstore"
Column | Type | Modifiers | Storage | Stats target | Description
----------+------------------------+-----------+----------+--------------+-------------
date | date | | plain | |
ref_id | character varying(256) | | extended | |
visits | hstore | | extended | |
pages | hstore | | extended | |
duration | hstore | | extended | |
Has OIDs: no
Tablespace: "tbs_data"
dev=# select count(*) from weekly_non_hstore;
count
----------
71818882
(1 row)
dev=# select count(*) from weekly_hstore;
count
---------
1292314
(1 row)
Query
dev=# explain analyze select cha_type,sum(visits) from weekly_non_hstore a join seg1 b on a.ref_id=b.ref_id group by cha_type order by sum(visits) desc;
QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------------------------------------
Sort (cost=3674073.37..3674431.16 rows=143115 width=27) (actual time=47520.637..47969.658 rows=3639539 loops=1)
Sort Key: (sum(a.visits))
Sort Method: quicksort Memory: 391723kB
-> HashAggregate (cost=3660386.70..3661817.85 rows=143115 width=27) (actual time=43655.637..44989.202 rows=3639539 loops=1)
-> Hash Join (cost=12029.58..3301286.54 rows=71820032 width=27) (actual time=209.789..26477.652 rows=36962761 loops=1)
Hash Cond: ((a.ref_id)::text = (b.ref_id)::text)
-> Seq Scan on weekly_non_hstore a (cost=0.00..1852856.32 rows=71820032 width=75) (actual time=0.053..8858.594 rows=71818882 loops=1)
-> Hash (cost=7382.59..7382.59 rows=371759 width=47) (actual time=209.189..209.189 rows=371759 loops=1)
Buckets: 65536 Batches: 1 Memory Usage: 28951kB
-> Seq Scan on seg1 b (cost=0.00..7382.59 rows=371759 width=47) (actual time=0.014..64.695 rows=371759 loops=1)
Total runtime: 48172.405 ms
(11 rows)
Time: 48173.569 ms
dev=# explain analyze select cha_type, sum(visits) from (select (each(visits)).key as cha_type,(each(visits)).value::numeric as visits from weekly_hstore a join seg1 b on a.ref_id=b.ref_id )foo group by cha_type order by sum(visits) desc;
QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------------------------------------
Sort (cost=7599039.89..7599040.39 rows=200 width=64) (actual time=70424.561..70986.202 rows=3639539 loops=1)
Sort Key: (sum((((each(a.visits)).value)::numeric)))
Sort Method: quicksort Memory: 394779kB
-> HashAggregate (cost=7599030.24..7599032.24 rows=200 width=64) (actual time=59267.120..60502.647 rows=3639539 loops=1)
-> Hash Join (cost=12029.58..2022645.24 rows=371759000 width=184) (actual time=186.140..34619.879 rows=36962761 loops=1)
Hash Cond: ((a.ref_id)::text = (b.ref_id)::text)
-> Seq Scan on weekly_hstore a (cost=0.00..133321.14 rows=1292314 width=230) (actual time=0.107..416.741 rows=1292314 loops=1)
-> Hash (cost=7382.59..7382.59 rows=371759 width=47) (actual time=185.742..185.742 rows=371759 loops=1)
Buckets: 65536 Batches: 1 Memory Usage: 28951kB
-> Seq Scan on seg1 b (cost=0.00..7382.59 rows=371759 width=47) (actual time=0.016..62.123 rows=371759 loops=1)
Total runtime: 71177.675 ms
