Re: Are JOINs allowed with DELETE FROM

Hi Michael,

Thank you for the link to the documentation page. I forgot to mention that
we're still using version 7.3. When I checked the 7.3 documentation for
DELETE, there was no mention of being able to use fields from different
tables in a WHERE clause. This feature must have been added in a
subsequent release of PostgreSQL.

Gaetano & John: I *did* try your suggestion. However, there were so many
summary ID's returned (9810 to be exact) that the DELETE seemed to be
taking forever. Here's an actual SELECT query that I ran as a test:

vsa=# vacuum analyze verbose vsa.tbl_win_patch_scan; [This is the
"summary" table from my abstracted example]
INFO: --Relation vsa.tbl_win_patch_scan--
INFO: Pages 374: Changed 0, Empty 0; Tup 10485: Vac 0, Keep 0, UnUsed 0.
Total CPU 0.01s/0.00u sec elapsed 0.00 sec.
INFO: --Relation pg_toast.pg_toast_39384--
INFO: Pages 62679: Changed 0, Empty 0; Tup 254116: Vac 0, Keep 0, UnUsed
0.
Total CPU 0.86s/0.21u sec elapsed 13.79 sec.
INFO: Analyzing vsa.tbl_win_patch_scan
VACUUM
Time: 18451.32 ms

vsa=# vacuum analyze verbose vsa.tbl_win_patch_scan_item; [This is the
"detail" table from my abstracted example]
INFO: --Relation vsa.tbl_win_patch_scan_item--
INFO: Pages 110455: Changed 0, Empty 0; Tup 752066: Vac 0, Keep 0, UnUsed
0.
Total CPU 2.23s/0.45u sec elapsed 42.07 sec.
INFO: --Relation pg_toast.pg_toast_39393--
INFO: Pages 2464: Changed 0, Empty 0; Tup 14780: Vac 0, Keep 0, UnUsed 0.
Total CPU 0.02s/0.02u sec elapsed 2.31 sec.
INFO: Analyzing vsa.tbl_win_patch_scan_item
VACUUM
Time: 62075.52 ms

vsa=# explain analyze SELECT * FROM vsa.tbl_win_patch_scan_item WHERE
win_patch_scan_id IN (SELECT id FROM vsa.tbl_win_patch_scan WHERE
scan_datetime < '2004-09-01 00:00:00');
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------------
Seq Scan on tbl_win_patch_scan_item (cost=0.00..379976970.68 rows=376033
width=1150) (actual time=11.50..27373.29 rows=62 loops=1)
Filter: (subplan)
SubPlan
-> Materialize (cost=505.06..505.11 rows=4 width=4) (actual
time=0.00..0.00 rows=2 loops=752066)
-> Seq Scan on tbl_win_patch_scan (cost=0.00..505.06 rows=4
width=4) (actual time=0.03..11.16 rows=2 loops=1)
Filter: (scan_datetime < '2004-09-01 00:00:00'::timestamp
without time zone)
Total runtime: 27373.65 msec
(7 rows)

Time: 27384.12 ms

I put in a very early date (2004-09-01) because I knew there would be very
few rows to look at (2 rows in vsa.tbl_win_patch_scan meet the date
criteria, and a total of 62 rows in vsa.tbl_win_patch_scan_item match
either of the two tbl_win_patch_scan ID's returned in the WHERE subquery).
Can anyone see a way of optimizing this so that it runs faster? The real
date I should be using is 2004-12-06 (~60 days retention), and when I do
use it, the query seems to take forever. I ran a number explan analyzes
with different scan_datetimes, and it seems that the execution time
increases exponentially with the number of rows (ID's) returned by the
subquery. Running top shows that the EXPLAIN is entirely CPU-bound. There
is no disk I/O during any query execution:

DATE=2004-09-01; SUMMARY ROWS=2; DETAIL ROWS=62; TIME=27.37 sec (Included
initial query cache loading effect)
vsa=# explain analyze SELECT * FROM vsa.tbl_win_patch_scan_item WHERE
win_patch_scan_id IN (SELECT id FROM vsa.tbl_win_patch_scan WHERE
scan_datetime < '2004-09-01 00:00:00');
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------------
Seq Scan on tbl_win_patch_scan_item (cost=0.00..379976970.68 rows=376033
width=1150) (actual time=11.50..27373.29 rows=62 loops=1)
Filter: (subplan)
SubPlan
-> Materialize (cost=505.06..505.11 rows=4 width=4) (actual
time=0.00..0.00 rows=2 loops=752066)
-> Seq Scan on tbl_win_patch_scan (cost=0.00..505.06 rows=4
width=4) (actual time=0.03..11.16 rows=2 loops=1)
Filter: (scan_datetime < '2004-09-01 00:00:00'::timestamp
without time zone)
Total runtime: 27373.65 msec
(7 rows)

Time: 27384.12 ms

DATE=2004-09-02; SUMMARY ROWS=2; DETAIL ROWS=62; TIME=8.26 sec
vsa=# explain analyze SELECT * FROM vsa.tbl_win_patch_scan_item WHERE
win_patch_scan_id IN (SELECT id FROM vsa.tbl_win_patch_scan WHERE
scan_datetime < '2004-09-02 00:00:00');
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------
Seq Scan on tbl_win_patch_scan_item (cost=0.00..380115740.80 rows=376033
width=1142) (actual time=10.42..8259.79 rows=62 loops=1)
Filter: (subplan)
SubPlan
-> Materialize (cost=505.06..505.48 rows=41 width=4) (actual
time=0.00..0.00 rows=2 loops=752066)
-> Seq Scan on tbl_win_patch_scan (cost=0.00..505.06 rows=41
width=4) (actual time=0.02..10.08 rows=2 loops=1)
Filter: (scan_datetime < '2004-09-02 00:00:00'::timestamp
without time zone)
Total runtime: 8259.91 msec
(7 rows)

Time: 8263.52 ms

DATE=2004-09-05; SUMMARY ROWS=3; DETAIL ROWS=93; TIME=5.61 sec
vsa=# explain analyze SELECT * FROM vsa.tbl_win_patch_scan_item WHERE
win_patch_scan_id IN (SELECT id FROM vsa.tbl_win_patch_scan WHERE
scan_datetime < '2004-09-05 00:00:00');
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------
Seq Scan on tbl_win_patch_scan_item (cost=0.00..380531977.65 rows=376033
width=1142) (actual time=10.11..5616.68 rows=93 loops=1)
Filter: (subplan)
SubPlan
-> Materialize (cost=505.06..506.58 rows=152 width=4) (actual
time=0.00..0.00 rows=3 loops=752066)
-> Seq Scan on tbl_win_patch_scan (cost=0.00..505.06 rows=152
width=4) (actual time=0.02..10.05 rows=3 loops=1)
Filter: (scan_datetime < '2004-09-05 00:00:00'::timestamp
without time zone)
Total runtime: 5616.81 msec
(7 rows)

Time: 5617.87 ms

DATE=2004-09-15; SUMMARY ROWS=16; DETAIL ROWS=674; TIME=18.03 sec
vsa=# explain analyze SELECT * FROM vsa.tbl_win_patch_scan_item WHERE
win_patch_scan_id IN (SELECT id FROM vsa.tbl_win_patch_scan WHERE
scan_datetime < '2004-09-15 00:00:00');
QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------------------
Seq Scan on tbl_win_patch_scan_item (cost=0.00..381919433.78 rows=376033
width=1142) (actual time=10.18..18032.25 rows=674 loops=1)
Filter: (subplan)
SubPlan
-> Materialize (cost=505.06..510.27 rows=521 width=4) (actual
time=0.00..0.01 rows=16 loops=752066)
-> Seq Scan on tbl_win_patch_scan (cost=0.00..505.06 rows=521
width=4) (actual time=0.02..10.11 rows=16 loops=1)
Filter: (scan_datetime < '2004-09-15 00:00:00'::timestamp
without time zone)
Total runtime: 18032.72 msec
(7 rows)

Time: 18033.78 ms

DATE=2004-09-16; SUMMARY ROWS=25; DETAIL ROWS=1131; TIME=26.22 sec
vsa=# explain analyze SELECT * FROM vsa.tbl_win_patch_scan_item WHERE
win_patch_scan_id IN (SELECT id FROM vsa.tbl_win_patch_scan WHERE
scan_datetime < '2004-09-16 00:00:00');
QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------------------
Seq Scan on tbl_win_patch_scan_item (cost=0.00..382058179.39 rows=376033
width=1142) (actual time=6.14..26218.56 rows=1131 loops=1)
Filter: (subplan)
SubPlan
-> Materialize (cost=505.06..510.64 rows=558 width=4) (actual
time=0.00..0.01 rows=25 loops=752066)
-> Seq Scan on tbl_win_patch_scan (cost=0.00..505.06 rows=558
width=4) (actual time=0.01..6.09 rows=25 loops=1)
Filter: (scan_datetime < '2004-09-16 00:00:00'::timestamp
without time zone)
Total runtime: 26219.24 msec
(7 rows)

Time: 26220.44 ms

DATE=2004-09-17; SUMMARY ROWS=34; DETAIL ROWS=1588; TIME=34.97 sec
vsa=# explain analyze SELECT * FROM vsa.tbl_win_patch_scan_item WHERE
win_patch_scan_id IN (SELECT id FROM vsa.tbl_win_patch_scan WHERE
scan_datetime < '2004-09-17 00:00:00');
QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------------------
Seq Scan on tbl_win_patch_scan_item (cost=0.00..382196925.01 rows=376033
width=1142) (actual time=10.25..34965.95 rows=1588 loops=1)
Filter: (subplan)
SubPlan
-> Materialize (cost=505.06..511.01 rows=595 width=4) (actual
time=0.00..0.02 rows=34 loops=752066)
-> Seq Scan on tbl_win_patch_scan (cost=0.00..505.06 rows=595
width=4) (actual time=0.02..10.16 rows=34 loops=1)
Filter: (scan_datetime < '2004-09-17 00:00:00'::timestamp
without time zone)
Total runtime: 34966.90 msec
(7 rows)

Time: 34967.95 ms

What I may end up doing is using the scripting language PHP to solve the
issue by running one query just to return the summary table ID's, and then
DELETE all the rows matching each ID individually by looping through the
ID's. I was looking for something more elegant, but this will work if its
the only solution.

Thank you all for your help with this.
--- Steve

___________________________________________________________________________________

Steven Rosenstein
IT Architect/Developer | IBM Virtual Server Administration
Voice/FAX: 845-689-2064 | Cell: 646-345-6978 | Tieline: 930-6001
Text Messaging: 6463456978 @ mobile.mycingular.com
Email: srosenst @ us.ibm.com

"Learn from the mistakes of others because you can't live long enough to
make them all yourself." -- Eleanor Roosevelt

Michael Fuhr
<mike(at)fuhr(dot)org>
To
02/06/2005 12:50 Steven Rosenstein/New
PM York/IBM(at)IBMUS
cc
pgsql-performance(at)postgresql(dot)org
Subject
Re: [PERFORM] Are JOINs allowed
with DELETE FROM

On Sun, Feb 06, 2005 at 12:16:13PM -0500, Steven Rosenstein wrote:
>
> DELETE FROM detail JOIN summary ON (summary.id=detail.sum_id) WHERE
> collect_date='2005-02-05';
>
> But I keep getting a parser error. Am I not allowed to use JOINs in a
> DELETE statement, or am I just fat-fingering the SQL text somewhere.

See the documentation for DELETE:

http://www.postgresql.org/docs/8.0/static/sql-delete.html

If you intend to delete the date's record from the summary table,
then the detail table could use a foreign key constraint defined
with ON DELETE CASCADE. Deleting a record from summary would then
automatically delete all associated records in detail.

--
Michael Fuhr
http://www.fuhr.org/~mfuhr/