Query performance concerning count

Question

Hi
I join three tables based on indexed fields.
Two tables have 25000 records, the third has only
a couple of records. The result of the join is
25000 records.
The query runs in 0.2 seconds. Very fast.
I try to count the records replacing the select ID
with count(ID) or with count(*) and it takes 2 seconds
to count the rows.
Any ideas ?
Thank you !!

Answer 1

COUNT(*) can most certainly make use of indexes, assuming you're using the cost-based optimizer and you have gathered statistics recently.
First, let's walk through what happens when there are no statistics on a table
scott@jcave > ANALYZE TABLE my_table DELETE STATISTICS;
scott@jcave > desc my_table;
Name                  Null?    Type
MYKEY                 NOT NULL NUMBER
VALUE                          VARCHAR2(100)
scott@jcave > set autotrace on;
scott@jcave > select count(*) from my_table;
  COUNT(*)
    227610
Elapsed: 00:00:03.03
Execution Plan
   0      SELECT STATEMENT Optimizer=CHOOSE
   1    0   SORT (AGGREGATE)
   2    1     TABLE ACCESS (FULL) OF 'MY_TABLE'
Statistics
         36  recursive calls
          0  db block gets
       1145  consistent gets
       1126  physical reads
        480  redo size
        381  bytes sent via SQL*Net to client
        503  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processedSo, it took 3 seconds to full-scan the table when there were no statistics. Now, lets gather some statistics and try again. Note that I'll be using the ANALYZE command, but production databases should probably be gathering statistics with the dbms_stats package regularly.
scott@jcave > analyze table my_table compute statistics;
Table analyzed.
Elapsed: 00:00:09.07
scott@jcave > analyze table my_table compute statistics for all indexes;
Table analyzed.
Elapsed: 00:00:04.00
scott@jcave > select count(*) from my_table;
  COUNT(*)
    227610
Elapsed: 00:00:00.08
Execution Plan
   0      SELECT STATEMENT Optimizer=CHOOSE (Cost=43 Card=1)
   1    0   SORT (AGGREGATE)
   2    1     INDEX (FAST FULL SCAN) OF 'SYS_C003704' (UNIQUE) (Cost=4
          3 Card=227610)
Statistics
         55  recursive calls
          2  db block gets
        517  consistent gets
        489  physical reads
        120  redo size
        381  bytes sent via SQL*Net to client
        503  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processedWe've gone down to a fraction of a second, now that the CBO knows that the index will be useful.
The moral of the story is that if you haven't gathered statistics recently, you may want to try doing so.
Justin
Distributed Database Consulting, Inc.
www.ddbcinc.com/askDDBC