Thick indexes - a look at count(1) query - Mailing list pgsql-hackers

Hi,<br />   I have submitted a new patch against thick indexes(indexes with snapshot)<br /><a
href="http://archives.postgresql.org/pgsql-hackers/2007-10/msg00220.php">http://archives.postgresql.org/pgsql-hackers/2007-10/msg00220.php.
</a><br/><br />I did look closely at improving the performance of count(1) queries. It worked well, when we are
selectinga sub-set of the result-set. But when i did a query like "select count(1) from table", it showed a improvement
inresponse time, but not to the extent, i wanted it to be. Let's have a look at the stats. <br /><br />gokul=# explain
analyzeselect count(1) from dd;<br />LOG:  EXECUTOR STATISTICS<br />DETAIL:  ! system usage stats:<br />        !      
0.277208elapsed 0.275457 user 0.000148 system sec<br />        !       [1.128422 user 0.004976 sys total]<br />       
!      0/0 [0/0] filesystem blocks in/out<br />        !       0/0 [0/0] page faults/reclaims, 0 [0] swaps<br />       
!      0 [0] signals rcvd, 0/0 [6/12] messages rcvd/sent<br />        !       0/39 [5/160] voluntary/involuntary
contextswitches <br />        ! buffer usage stats:<br />        !       Shared blocks:       1024 Logical
Reads,         0 Physical Reads,          0 written, buffer hit rate = 100.00%<br />        !       Local 
blocks:         0 read,          0 written, buffer hit rate = 0.00%<br />        !       Direct blocks:          0
read,         0 written<br />STATEMENT:  explain analyze select count(1) from dd;<br
/>                                                           QUERY
PLAN                                                           <br
/>----------------------------------------------------------------------------------------------------------------------------------<br
/> Aggregate (cost=7323.10..7323.11 rows=1 width=0) (actual time=276.838..276.838 rows=1 loops=1) <br />   ->  Index
OnlyScan using idx on dd  (cost=0.00..6741.42 rows=232671 width=0) (actual time=0.042..160.753 rows=232679 loops=1)<br
/> Totalruntime: 276.928 ms<br />(3 rows)<br /><br />gokul=# set enable_indexscan=off;<br /> SET<br /><br />gokul=#
explainanalyze select count(1) from dd;<br />LOG:  EXECUTOR STATISTICS<br />DETAIL:  ! system usage stats:<br />       
!      0.331441 elapsed 0.258903 user 0.067953 system sec<br />        !       [1.906069 user 0.211479 sys total]<br
/>       !       0/0 [0/0] filesystem blocks in/out<br />        !       0/0 [0/0] page faults/reclaims, 0 [0] swaps<br
/>       !       0 [0] signals rcvd, 0/0 [10/19] messages rcvd/sent<br />        !       0/39 [9/312]
voluntary/involuntarycontext switches <br />        ! buffer usage stats:<br />        !       Shared blocks:      
5223Logical Reads,       4391 Physical Reads,          0 written, buffer hit rate = 15.93%<br />        !       Local 
blocks:         0 read,          0 written, buffer hit rate = 0.00%<br />        !       Direct blocks:          0
read,         0 written<br />STATEMENT:  explain analyze select count(1) from dd;<br
/>                                                  QUERY PLAN                                                    <br
/>-----------------------------------------------------------------------------------------------------------------<br
/> Aggregate (cost=8131.39..8131.40 rows=1 width=0) (actual time=331.075..331.076 rows=1 loops=1)<br />    ->  Seq
Scanon dd  (cost=0.00..7549.71 rows=232671 width=0) (actual time=0.042..203.958 rows=232679 loops=1)<br /> Total
runtime:331.167 ms<br />(3 rows)<br /><br />I have run the query multiple times and it shows the response time, around
whatis shown here <br />The table is just a multiple copy of pg_class table( approx 200000 rows). As it can be seen,
theLogical reads show a ratio of 1:5, but the  response time is not in the same ratio. I tried to profile and couldn't
findanything significant. Eventhough it shows 4391 physical reads, that's from OS cache,  since i ave already run the
querymultiple times. <br /><br />One more disadvantage with using select count(1) using index scan is that, it pollutes
theshared memory, unlike full-table scans. But something can be done in the regard.<br /><br /><br /><br />Any
thoughts?<br/><br />Thanks, <br />Gokul.<br />