Hi Pavel,
thanks for the advice on how to uncorrelate the query. I must admit I
didn't know about the UPDATE ... SET ... FROM ... syntax.
Now the UPDATE runs in an acceptable time inside a transaction,
however the query plan still differs when I run it outside.
Outside a transaction:
QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------------------------
Merge Join (cost=0.00..11481.84 rows=65756 width=1087) (actual
time=0.151..323.856 rows=65756 loops=1)
Merge Cond: (_struct.id = tmp.id)
-> Index Scan using "_PK_struct" on _struct (cost=0.00..7084.50
rows=98149 width=1083) (actual time=0.028..137.463 rows=32300 loops=1)
-> Index Scan using idx_tmp__id on tmp (cost=0.00..3330.02
rows=65756 width=12) (actual time=0.115..58.601 rows=65756 loops=1)
Total runtime: 2905.580 ms
This looks like an optimal plan and average run time over 5 runs is
2660 ms.
Inside a transaction:
QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------------------------
Merge Join (cost=7427.63..16159.84 rows=65756 width=1087) (actual
time=315.570..574.075 rows=65756 loops=1)
Merge Cond: (_struct.id = tmp.id)
-> Index Scan using "_PK_struct" on _struct (cost=0.00..7500.50
rows=98149 width=1083) (actual time=0.020..129.915 rows=32300 loops=1)
-> Sort (cost=7427.63..7592.02 rows=65756 width=12) (actual
time=315.538..333.359 rows=65756 loops=1)
Sort Key: tmp.id
Sort Method: quicksort Memory: 4617kB
-> Seq Scan on tmp (cost=0.00..2165.56 rows=65756
width=12) (actual time=10.070..37.411 rows=65756 loops=1)
Trigger for constraint _FK_struct_2_collection: time=1105.892
calls=32300
Trigger for constraint _FK_struct_2_text: time=1468.009 calls=32300
Total runtime: 4955.784 ms
Again, the planner does not use the index on tmp (id) although I put
an "ANALYZE tmp" right before the UPDATE. Average run time over 5
runs is 4610 ms.
Thanks,
Viktor
Am 16.07.2008 um 15:33 schrieb Pavel Stehule:
Hello
my advice is little bit offtopic, I am sorry. Why you use correlated
subquery? Your update statement should be
update _struct set left_token = tmp.left_token from tmp where
_struct.id = tmp.id;
send output of explain analyze statement, please. etc
explain analyze UPDATE _struct SET left_token = (SELECT DISTINCT
left_token FROM tmp WHERE _struct.id = tmp.id)
regards
Pavel Stehule
2008/7/16 Viktor Rosenfeld <rosenfel@xxxxxxxxxxxxxxxxxxxxxxx>:
Hi Tom,
Postgres is indeed selecting a bad plan. Turns out that the index
I created
to speed up the UPDATE isn't used inside a transaction block.
Here's the plan for "UPDATE _struct SET left_token = (SELECT DISTINCT
left_token FROM tmp WHERE _struct.id = tmp.id)" outside of a
transaction:
QUERY PLAN
-------------------------------------------------------------------------------------------
Seq Scan on _struct (cost=0.00..826643.13 rows=98149 width=1083)
SubPlan
-> Unique (cost=8.38..8.40 rows=1 width=4)
-> Sort (cost=8.38..8.39 rows=4 width=4)
Sort Key: tmp.left_token
-> Index Scan using idx_tmp__id on tmp
(cost=0.00..8.34
rows=4 width=4)
Index Cond: ($0 = id)
And inside a transaction:
QUERY PLAN
---------------------------------------------------------------------------------------
Seq Scan on _struct (cost=100000000.00..3230175260746.00 rows=32300
width=70)
SubPlan
-> Unique (cost=100002329.99..100002330.01 rows=1 width=4)
-> Sort (cost=100002329.99..100002330.00 rows=4 width=4)
Sort Key: tmp.left_token
-> Seq Scan on tmp (cost=100000000.00..100002329.95
rows=4 width=4)
Filter: ($0 = id)
The high cost of the seqscan on tmp are because I tried disabling
sequential
scans inside the transaction to force an index scan, which Postgres
decided
to ignore in this case.
Putting an ANALYZE tmp and ANALYZE _struct right before the UPDATE
didn't
help either. (Also shouldn't the creation of an index on tmp (id)
take care
of analyzing that column?)
Thanks,
Viktor
Am 14.07.2008 um 20:52 schrieb Tom Lane:
Viktor Rosenfeld <rosenfel@xxxxxxxxxxxxxxxxxxxxxxx> writes:
the script below runs very fast when executed alone. But when I call
it from within a transaction block it's so slow that I have to abort
it after a while. Specifically the second-to-last UPDATE seems to
take forever within a transaction while it completes in about 3
seconds outside a transaction.
Since the table you're working on was just created in the same
transaction, there's been no opportunity for autovacuum to run an
ANALYZE on it; that's probably preventing selection of a good plan.
Try throwing in an "ANALYZE tmp" after you load the table.
regards, tom lane
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