Oops, forgot to mention:
PostgreSQL 8.0 beta 2 Windows.
Thanks,
Gary.
On 8 Oct 2004 at 20:32, Gary Doades wrote:
>
> I'm looking at one of my standard queries and have encountered some strange
> performance
> problems.
>
> The query below is to search for vacant staff member date/time slots given a series
> of target
> date/times. The data contained in the booking_plan/staff_booking tables contain the
> existing
> bookings, so I'm looking for "clashing" bookings to eliminate them from a candidate
> list.
>
> The query is:
>
> select distinct b.staff_id from staff_booking b, booking_plan bp,
> t_search_reqt_dates rd
> where b.booking_id = bp.booking_id
> and rd.datetime_from <= bp.datetime_to and rd.datetime_to >= bp.datetime_from
> AND bp.booking_date between rd.reqt_date-1 and rd.reqt_date+1
> and rd.search_id = 13
> and rd.reqt_date between '2004-09-30' AND '2005-12-31'
>
> There are 197877 rows in staff_booking, 573416 rows in booking_plan and 26 rows in
> t_search_reqt_dates.
>
> The t_search reqt_dates is a temp table created and populated with the target
> date/times. The
> temp table is *not* analyzed, all the other are.
>
> The "good" query plan comes with the criteria on search_id and reqt_date given in
> the last two
> lines in the query. Note all the rows in the temp table are search_id = 13 and all
> the rows are
> between the two dates, so the whole 26 rows is always pulled out.
>
> In this case it is doing exactly what I expect. It is pulling all rows from the
> t_search_reqt_dates
> table, then pulling the relevant records from the booking_plan and then hashing with
> staff_booking. Excellent performance.
>
> The problem is I don't need the clauses for search_id and reqt_dates as the whole
> table is
> always read anyway. The good plan is because the planner thinks just one row will be
> read from
> t_search_reqt_dates.
>
> If I remove the redundant clauses, the planner now estimates 1000 rows returned from
> the table,
> not unreasonable since it has no statistics. But *why* in that case, with *more*
> estimated rows
> does it choose to materialize that table (26 rows) 573416 times!!!
>
> whenever it estimates more than one row it chooses the bad plan.
>
> I really want to remove the redundant clauses, but I can't. If I analyse the table,
> then it knows
> there are 26 rows and chooses the "bad" plan whatever I do.
>
> Any ideas???
>
> Cheers,
> Gary.
>
> -------------------- Plans for above query ------------------------
>
> Good QUERY PLAN
> Unique (cost=15440.83..15447.91 rows=462 width=4) (actual time=1342.000..1342.000
> rows=110 loops=1)
> -> Sort (cost=15440.83..15444.37 rows=7081 width=4) (actual time=1342.000..1342.000
> rows=2173 loops=1)
> Sort Key: b.staff_id
> -> Hash Join (cost=10784.66..15350.26 rows=7081 width=4) (actual
> time=601.000..1331.000 rows=2173 loops=1)
> Hash Cond: ("outer".booking_id = "inner".booking_id)
> -> Seq Scan on staff_booking b (cost=0.00..4233.39 rows=197877 width=8) (actual
> time=0.000..400.000 rows=197877 loops=1)
> -> Hash (cost=10781.12..10781.12 rows=7080 width=4) (actual
> time=591.000..591.000 rows=0 loops=1)
> -> Nested Loop (cost=0.00..10781.12 rows=7080 width=4) (actual
> time=10.000..581.000 rows=2173 loops=1)
> Join Filter: (("outer".datetime_from <= "inner".datetime_to) AND
> ("outer".datetime_to >= "inner".datetime_from))
> -> Seq Scan on t_search_reqt_dates rd (cost=0.00..16.50 rows=1 width=20)
> (actual time=0.000..0.000 rows=26 loops=1)
> Filter: ((search_id = 13) AND (reqt_date >= '2004-09-30'::date) AND
> (reqt_date <= '2005-12-31'::date))
> -> Index Scan using booking_plan_idx2 on booking_plan bp
> (cost=0.00..10254.91 rows=63713 width=24) (actual time=0.000..11.538 rows=5871
> loops=26)
> Index Cond: ((bp.booking_date >= ("outer".reqt_date - 1)) AND
> (bp.booking_date <= ("outer".reqt_date + 1)))
> Total runtime: 1342.000 ms
>
>
> Bad QUERY PLAN
> Unique (cost=7878387.29..7885466.50 rows=462 width=4) (actual
> time=41980.000..41980.000
> rows=110 loops=1)
> -> Sort (cost=7878387.29..7881926.90 rows=7079211 width=4) (actual
> time=41980.000..41980.000 rows=2173 loops=1)
> Sort Key: b.staff_id
> -> Nested Loop (cost=5314.32..7480762.73 rows=7079211 width=4) (actual
> time=6579.000..41980.000 rows=2173 loops=1)
> Join Filter: (("inner".datetime_from <= "outer".datetime_to) AND
> ("inner".datetime_to >=
> "outer".datetime_from) AND ("outer".booking_date >= ("inner".reqt_date - 1)) AND
> ("outer".booking_date <= ("inner".reqt_date + 1)))
> -> Hash Join (cost=5299.32..26339.73 rows=573416 width=24) (actual
> time=2413.000..7832.000 rows=573416 loops=1)
> Hash Cond: ("outer".booking_id = "inner".booking_id)
> -> Seq Scan on booking_plan bp (cost=0.00..7646.08 rows=573416 width=24)
> (actual time=0.000..1201.000 rows=573416 loops=1)
> -> Hash (cost=4233.39..4233.39 rows=197877 width=8) (actual
> time=811.000..811.000 rows=0 loops=1)
> -> Seq Scan on staff_booking b (cost=0.00..4233.39 rows=197877 width=8)
> (actual time=0.000..430.000 rows=197877 loops=1)
> -> Materialize (cost=15.00..20.00 rows=1000 width=20) (actual time=0.001..0.019
> rows=26 loops=573416)
> -> Seq Scan on t_search_reqt_dates rd (cost=0.00..15.00 rows=1000 width=20)
> (actual time=0.000..0.000 rows=26 loops=1)
> Total runtime: 41980.000 ms
>
---------------------------(end of broadcast)---------------------------
TIP 8: explain analyze is your friend
