Re: difference in plan between 8.0 and 8.1? - Mailing list pgsql-performance

Tom Lane wrote:
> Alan Stange <stange@rentec.com> writes:
>
>>  Unique  (cost=2717137.08..2771407.21 rows=10854026 width=8)
>>    ->  Sort  (cost=2717137.08..2744272.14 rows=10854026 width=8)
>>          Sort Key: timeseriesid
>>          ->  Bitmap Heap Scan on tbltimeseries
>> (cost=48714.09..1331000.42 rows=10854026 width=8)
>>                Recheck Cond: (timeseriesid > 0)
>>                ->  Bitmap Index Scan on idx_timeseris
>> (cost=0.00..48714.09 rows=10854026 width=0)
>>                      Index Cond: (timeseriesid > 0)
>> (7 rows)
>>
>
>
>> I'm hoping someone can explain the new query plan (as I'm not sure I
>> understand what it is doing).
>>
>
> The index scan is reading the index to find out which heap tuple IDs
> (TIDs) the index says meet the condition.  It returns a bitmap of the
> tuple locations (actually, an array of per-page bitmaps).  The heap
> scan goes and fetches the tuples from the table, working in TID order
> to avoid re-reading the same page many times, as can happen for ordinary
> index scans.  Since the result isn't sorted, we have to do a sort to get
> it into the correct order for the Unique step.
>
> Because it avoids random access to the heap, this plan can be a lot
> faster than a regular index scan.  I'm not sure at all that 8.1 is
> doing good relative cost estimation yet, though.  It would be
> interesting to see EXPLAIN ANALYZE results for both ways.  (You can
> use enable_bitmapscan and enable_indexscan to force the planner to pick
> the plan it thinks is slower.)
Just to be clear.  The index is on the timeseriesid column.   Also, We
usually have the where clause with some non-zero number.

Anyway, here's the basic query, with variations added on belowe:

fiasco=# explain analyze select timeseriesid from tbltimeseries where
timeseriesid > 0;
                                                                   QUERY
PLAN

------------------------------------------------------------------------------------------------------------------------------------------------
 Bitmap Heap Scan on tbltimeseries  (cost=48906.82..1332935.19
rows=10905949 width=8) (actual time=16476.337..787480.979 rows=10907853
loops=1)
   Recheck Cond: (timeseriesid > 0)
   ->  Bitmap Index Scan on idx_timeseris  (cost=0.00..48906.82
rows=10905949 width=0) (actual time=16443.585..16443.585 rows=10907853
loops=1)
         Index Cond: (timeseriesid > 0)
 Total runtime: 791340.341 ms
(5 rows)



Now add the order:

fiasco=# explain analyze select timeseriesid from tbltimeseries where
timeseriesid > 0 order by timeseriesid;

QUERY PLAN

------------------------------------------------------------------------------------------------------------------------------------------------------
 Sort  (cost=2726087.93..2753352.81 rows=10905949 width=8) (actual
time=821090.666..826353.054 rows=10913868 loops=1)
   Sort Key: timeseriesid
   ->  Bitmap Heap Scan on tbltimeseries  (cost=48912.82..1332941.19
rows=10905949 width=8) (actual time=16353.921..757075.349 rows=10913868
loops=1)
         Recheck Cond: (timeseriesid > 0)
         ->  Bitmap Index Scan on idx_timeseris  (cost=0.00..48912.82
rows=10905949 width=0) (actual time=16335.239..16335.239 rows=10913868
loops=1)
               Index Cond: (timeseriesid > 0)
 Total runtime: 830829.145 ms
(7 rows)




and the distinct:

fiasco=# explain analyze select distinct timeseriesid from tbltimeseries
where timeseriesid > 0 order by timeseriesid;

QUERY PLAN

------------------------------------------------------------------------------------------------------------------------------------------------------------
 Unique  (cost=2726087.93..2780617.68 rows=10905949 width=8) (actual
time=816938.970..831119.423 rows=10913868 loops=1)
   ->  Sort  (cost=2726087.93..2753352.81 rows=10905949 width=8) (actual
time=816938.967..822298.802 rows=10913868 loops=1)
         Sort Key: timeseriesid
         ->  Bitmap Heap Scan on tbltimeseries
(cost=48912.82..1332941.19 rows=10905949 width=8) (actual
time=15866.736..752851.006 rows=10913868 loops=1)
               Recheck Cond: (timeseriesid > 0)
               ->  Bitmap Index Scan on idx_timeseris
(cost=0.00..48912.82 rows=10905949 width=0) (actual
time=15852.652..15852.652 rows=10913868 loops=1)
                     Index Cond: (timeseriesid > 0)
 Total runtime: 835558.312 ms
(8 rows)




Now the usual query from 8.0:

fiasco=# set enable_bitmapscan=false; explain analyze select distinct
timeseriesid from tbltimeseries where timeseriesid > 0 order by
timeseriesid;
SET

QUERY PLAN

--------------------------------------------------------------------------------------------------------------------------------------------------------------
 Unique  (cost=0.00..14971276.10 rows=10905949 width=8) (actual
time=24.930..999645.638 rows=10913868 loops=1)
   ->  Index Scan using idx_timeseris on tbltimeseries
(cost=0.00..14944011.22 rows=10905949 width=8) (actual
time=24.926..989117.882 rows=10913868 loops=1)
         Index Cond: (timeseriesid > 0)
 Total runtime: 1003549.067 ms
(4 rows)




And now a sequential scan of the table itself:

fiasco=# set enable_bitmapscan=false; set enable_indexscan=false;
explain analyze select distinct timeseriesid from tbltimeseries where
timeseriesid > 0 order by timeseriesid;
SET
SET

QUERY PLAN

------------------------------------------------------------------------------------------------------------------------------------------------------
 Unique  (cost=102677188.75..102731718.49 rows=10905949 width=8) (actual
time=956783.989..971036.657 rows=10919883 loops=1)
   ->  Sort  (cost=102677188.75..102704453.62 rows=10905949 width=8)
(actual time=956783.985..962115.616 rows=10919883 loops=1)
         Sort Key: timeseriesid
         ->  Seq Scan on tbltimeseries  (cost=100000000.00..101284042.00
rows=10905949 width=8) (actual time=7.314..893267.030 rows=10919883 loops=1)
               Filter: (timeseriesid > 0)
 Total runtime: 975393.678 ms
(6 rows)


For us, the query is best served by the index scan as the ordering comes
for free and results can be streamed to a client immediately.  So, while
the whole query is a bit slower, the client can begin processing the
results immediately.    The client has three threads which stream in two
sets of id's and emit delete statements in smaller batches.   It can be
done as one statement, but on our production system that statement can
run for 10 hours and delete 20M rows...which conflicts with the vacuum
process.   This version can be throttled, stopped and restarted at any
time and no work is lost compared to a single long running query.