[PERFORM] Trouble managing planner for timestamptz columns
We have large tables that hold statistics based on time. They are of the form. CREATE TABLE stats ( id serial primary key, logtime timestamptz, d1 int, s1 bigint ); CREATE INDEX idx on stats(logtime); Some of these tables have new data inserted at a rate of 500,000+ rows / hour. The entire table will grow to being 10's to 100's of millions of rows in size. (Yes, we are also paritioning these, it's the size of an individual partition that we're talking about). We tend to analyze these tables every day or so and this doesn't always prove to be sufficient Our application is a reporting application and the end users typically like to query the newest data the most. As such, the queries of the form... select * from stats inner join dimension_d1 using (d1) where logtime between X and Y and d1.something = value; This usually results in a hash join (good thing) where the dimension table is loaded into the hash table and it index scans stats using idx index. The trouble starts when both X and Y are times after the last analyze. This restriction clause is outside the range of values in the historgram created by the last analyze. Postgres's estimate on the number of rows returned here is usually very low and incorrect, as you'd expect... Trouble can occur when the planner will flip its decision and decide to hash join by loading the results of the index scan on idx into the hash table instead of the dimension table Since the table is so large and the system is busy (disk not idle at all), doing an analyze on this table in the production system can take 1/2 hour! (statistics collector set to 100). We can't afford to analyze more often... It certainly would be nice if postgres could understand somehow that some columns are dynamic and that it's histogram could be stretched to the maximal values or some other technique for estimating rows to the right of the range of values in the histogram... Or have some concept of error bars on it's planner decisions Suggestions? Comments? Marc ---(end of broadcast)--- TIP 4: Have you searched our list archives? http://archives.postgresql.org
Re: [PERFORM] Trouble managing planner for timestamptz columns
Marc Morin [EMAIL PROTECTED] writes: We tend to analyze these tables every day or so and this doesn't always prove to be sufficient Seems to me you just stated your problem. Instead of having the planner make wild extrapolations, why not set up a cron job to analyze these tables more often? Or use autovacuum which will do it for you. Since the table is so large and the system is busy (disk not idle at all), doing an analyze on this table in the production system can take 1/2 hour! (statistics collector set to 100). I'd believe that for vacuum analyze, but analyze alone should be cheap. Have you perhaps got some weird datatypes in the table? Maybe you should back off the stats target a bit? We do support analyzing selected columns, so you might try something like a cron job analyzing only the timestamp column, with a suitably low stats target for that column. This would yield numbers far more reliable than any extrapolation the planner could do. regards, tom lane ---(end of broadcast)--- TIP 5: don't forget to increase your free space map settings
Re: [PERFORM] Trouble managing planner for timestamptz columns
Well this analyze just took 12 minutes... Stats target of 100. # time psql xxx xxx -c analyze elem_trafficstats_1 ANALYZE real12m1.070s user0m0.001s sys 0m0.015s A large table, but by far, not the largest... Have about 1 dozen or so tables like this, so analyzing them will take 3-4 hours of time... No weird datatypes, just bigints for facts, timestamptz and ints for dimensions. My problem is not the analyze itself, it's the fact that our db is really busy doing stuff Analyze I/O is competing... I am random I/O bound like crazy. If I set the stats target to 10, I get # time psql xxx -c set session default_statistics_target to 10;analyze elem_trafficstats_1 ANALYZE real2m15.733s user0m0.009s sys 0m2.255s Better, but not sure what side affect this would have. -Original Message- From: Tom Lane [mailto:[EMAIL PROTECTED] Sent: Friday, March 10, 2006 1:31 PM To: Marc Morin Cc: pgsql-performance@postgresql.org Subject: Re: [PERFORM] Trouble managing planner for timestamptz columns Marc Morin [EMAIL PROTECTED] writes: We tend to analyze these tables every day or so and this doesn't always prove to be sufficient Seems to me you just stated your problem. Instead of having the planner make wild extrapolations, why not set up a cron job to analyze these tables more often? Or use autovacuum which will do it for you. Since the table is so large and the system is busy (disk not idle at all), doing an analyze on this table in the production system can take 1/2 hour! (statistics collector set to 100). I'd believe that for vacuum analyze, but analyze alone should be cheap. Have you perhaps got some weird datatypes in the table? Maybe you should back off the stats target a bit? We do support analyzing selected columns, so you might try something like a cron job analyzing only the timestamp column, with a suitably low stats target for that column. This would yield numbers far more reliable than any extrapolation the planner could do. regards, tom lane ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster
