how to help the planner
| От | Marty Frasier |
|---|---|
| Тема | how to help the planner |
| Дата | |
| Msg-id | CAAX5s64o2LCF0cWNqDMTiM1JuSyyESvh1T1QW1cmDTpi4n2rdg@mail.gmail.com обсуждение исходный текст |
| Ответы |
Re: how to help the planner
(Tom Lane <tgl@sss.pgh.pa.us>)
Re: how to help the planner (Stephen Frost <sfrost@snowman.net>) |
| Список | pgsql-performance |
Greetings,
We've been using postgreSQL for a few years. This is my first post here and first real dive into query plans.
A description of what you are trying to achieve and what results you expect.: Query results of nested joins of table. Results are correct - just takes a long time with selected plan.
PostgreSQL version number you are running: PostgreSQL 9.2.3 on x86_64-unknown-linux-gnu, compiled by gcc (GCC) 4.4.6 20120305 (Red Hat 4.4.6-4), 64-bit
How you installed PostgreSQL:
yum, using PGDG repo (package postgresql92-server-9.2.3-2PGDG.rhel6.x86_64 and friends)
Changes made to the settings in the postgresql.conf file
DateStyle = ISO, MDY
default_tablespace = esc_data
default_text_search_config = pg_catalog.english
effective_cache_size = 24GB
lc_messages = en_US.UTF-8
lc_monetary = en_US.UTF-8
lc_numeric = en_US.UTF-8
lc_time = en_US.UTF-8
listen_addresses = 0.0.0.0
log_connections = on
log_destination = stderr
log_disconnections = on
log_line_prefix = %t %c
log_rotation_age = 1d
log_timezone = US/Eastern
logging_collector = on
maintenance_work_mem = 96MB
max_connections = 100
search_path = "$user", esc_funcs, public
shared_buffers = 8GB
TimeZone = US/Eastern
track_functions = all
track_io_timing = on
Operating system and version:
Red Hat Enterprise Linux Server release 6.4 (Santiago)
What program you're using to connect to PostgreSQL:
java(jdbc) and psql
Is there anything relevant or unusual in the PostgreSQL server logs?:
no
The issue is similar on PostgreSQL 9.0.5 on x86_64-pc-linux-gnu, compiled by GCC gcc-4.4.real (Ubuntu 4.4.3-4ubuntu5) 4.4.3 on Ubuntu 10.04 64-bit although we're doing troubleshooting on our new RHEL server.
We have a particular query that takes about 75 minutes to complete. The selected execution plan estimates 1 row from several of the outermost results so picks nested loop join resolutions. That turns out to be a bad choice since actual row counts are in the thirty to fifty thousand range.original selected plan: http://explain.depesz.com/s/muR
I set enable_nestloop=false to hint at the planner not to use nested loop. That resulted in 13 second runtime. It appears this plan was considered originally but estimated cost was higher than the plan above.
enable_nestloop=false: http://explain.depesz.com/s/mAa
We tried rewriting the query using WITH clauses. That took 82 seconds but plan thought it would take much longer.
using with clauses: http://explain.depesz.com/s/GEZ
We have been looking into the issue to the best of our ability but can't figure out how to help the planner. I've looked at the planner source some and see where row count is set to 1 if it's <= 1. I haven't found where it's set yet but presume it was unable to determine the result set row count and defaulted to 1.
I've run analyze manually and tried it with default_statistics_target=10000 to see if that helped. It didn't.
The table is static - no new rows are being added and there is no other load on the database.
schema dump: http://pastebin.com/pUU0BJvr
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