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>Hi,
>we are seeing cases of EXPLAIN INSERT INTO foo SELECT ... taking over an hour, with disk I/O utilization (percent of time device is busy) at 100% the whole time, although I/O bandwidth is not saturated. This is on PostgreSQL 9.1.13. >What could cause this? Note that there is no ANALYZE. Is it possible that the SELECT is actually executed, in planning the INSERT? >When executing the INSERT itself (not EXPLAIN) immediately afterwards, that logs a "temporary file" message, but the EXPLAIN invocation does not (though the disk I/O suggests that a large on-disk sort is occurring): >LOG: temporary file: path "base/pgsql_tmp/pgsql_tmp6016.0", size 744103936 >STATEMENT: INSERT INTO f_foo SELECT >[...] >During that actual execution, there's a lower disk I/O utilization (though a higher MB/s rate). >Charts of disk I/O utilization and rate are at http://postimg.org/image/628h6jmn3/ ... the solid 100% span is the EXPLAIN statement, ending at 6:13:30pm, followed by the INSERT statement ending at 6:32pm. Metrics are collected by New Relic; their definition of I/O utilization is at https://discuss.newrelic.com/t/disk-i-o-metrics/2900 >Here's the EXPLAIN statement: >LOG: duration: 3928778.823 ms statement: EXPLAIN INSERT INTO f_foo SELECT > t_foo.fk_d1, > t_foo.fk_d2, > t_foo.fk_d3, > t_foo.fk_d4, > t_foo.fk_d5, > t_foo.fk_d6, > t_foo.value >FROM t_foo >WHERE NOT (EXISTS > (SELECT * > FROM f_foo > WHERE f_foo.fk_d2 = t_foo.fk_d2 > AND f_foo.fk_d3 = t_foo.fk_d3 > AND f_foo.fk_d4 = t_foo.fk_d4 > AND f_foo.fk_d5 = t_foo.fk_d5 > AND f_foo.fk_d6 = t_foo.fk_d6 > AND f_foo.fk_d1 = t_foo.fk_d1)) >(where t_foo is a temp table previously populated using COPY, and the NOT EXISTS subquery refers to the same table we are inserting into) >Here's the EXPLAIN output: >Insert on f_foo (cost=8098210.50..9354519.69 rows=1 width=16) > -> Merge Anti Join (cost=8098210.50..9354519.69 rows=1 width=16) > Merge Cond: ((t_foo.fk_d2 = public.f_foo.fk_d2) AND > (t_foo.fk_d3 = public.f_foo.fk_d3) AND > (t_foo.fk_d4 = public.f_foo.fk_d4) AND > (t_foo.fk_d5 = public.f_foo.fk_d5) AND > (t_foo.fk_d6 = public.f_foo.fk_d6) AND > (t_foo.fk_d1 = public.f_foo.fk_d1)) > -> Sort (cost=3981372.25..4052850.70 rows=28591380 width=16) > Sort Key: t_foo.fk_d2, t_foo.fk_d3, t_foo.fk_d4, t_foo.fk_d5, > t_foo.fk_d6, t_foo.fk_d1 > -> Seq Scan on t_foo (cost=0.00..440461.80 rows=28591380 > width=16) > -> Sort (cost=4116838.25..4188025.36 rows=28474842 width=12) > Sort Key: public.f_foo.fk_d2, public.f_foo.fk_d3, > public.f_foo.fk_d4, public.f_foo.fk_d5, > public.f_foo.fk_d6, public.f_foo.fk_d1 > -> Seq Scan on f_foo (cost=0.00..591199.42 rows=28474842 > width=12) >The INSERT is indeed rather large (which is why we're issuing an EXPLAIN ahead of it to log the plan). So its long execution time is expected. But I want to understand why the EXPLAIN takes even longer. >The table looks like this: >\d f_foo >Table "public.f_foo" > Column | Type | Modifiers >--------+----------+----------- > fk_d1 | smallint | not null > fk_d2 | smallint | not null > fk_d3 | smallint | not null > fk_d4 | smallint | not null > fk_d5 | smallint | not null > fk_d6 | smallint | not null > value | integer | >Indexes: > "f_foo_pkey" PRIMARY KEY, btree (fk_d2, fk_d6, fk_d4, fk_d3, fk_d5, fk_d1) CLUSTER > "ix_f_foo_d4" btree (fk_d4) > "ix_f_foo_d3" btree (fk_d3) > "ix_f_foo_d5" btree (fk_d5) > "ix_f_foo_d6" btree (fk_d6) >Foreign-key constraints: > "f_foo_d2_fkey" FOREIGN KEY (fk_d2) REFERENCES d2(id) DEFERRABLE > "f_foo_d6_fkey" FOREIGN KEY (fk_d6) REFERENCES d6(id) DEFERRABLE > "f_foo_d5_fkey" FOREIGN KEY (fk_d5) REFERENCES d5(id) DEFERRABLE > "f_foo_d4_fkey" FOREIGN KEY (fk_d4) REFERENCES d4(id) DEFERRABLE > "f_foo_d3_fkey" FOREIGN KEY (fk_d3) REFERENCES d3(id) DEFERRABLE >Conceivably relevant (though I don't know how): this database has a very large number of table objects (1.3 million rows in pg_class). But other EXPLAINs are not taking anywhere near this long in this DB; the heavy EXPLAIN is only seen on INSERT into this and a couple of other tables with tens of millions of rows. >Any ideas? >Thanks, best regards, >- Gulli > Hi, I've no clue for the time required by EXPLAIN but some more information are probably relevant to find an explanation: - postgres version - number of rows inserted by the query - how clean is your catalog in regard to vacuum ( can you run vacuum full verbose & analyze it, and then retry the analyze statement ?) - any other process that may interfere, e.g. while locking some catalog tables ? - statistic target ? - is your temp table analyzed? - any index on it ? We have about 300'000 entries in our pg_class tables, and I've never seen such an issue. regards, Marc Mamin |
