Noob in SQL Tuning seeking help

Question

I am trying to learn SQL Tuning and the workings of it. I know the first step is probably explain execution plan of the sql statement? My problem is I do not understand what am I suppose to gather from the execution plan.
SQL> set autotrace traceonly explain
SQL> select * from employees A
     where A.last_employed =
     ( SELECT TO_CHAR(MAX(TO_DATE(PAY_DATE,'MMRR')),'MMYY')
       FROM PAYROLL_VW1 B
       WHERE B.SSN = A.SSN AND B.PAY_TYPE_CODE IN ('02','12','22','32')
      AND STATUS = 'ACTIVE' AND A.BRANCH=B.BRANCH
Execution Plan
   0      SELECT STATEMENT Optimizer=ALL_ROWS (Cost=47427 Card=1 Bytes
          =180)
   1    0   FILTER
   2    1     TABLE ACCESS (FULL) OF 'EMPLOYEES' (TABLE) (Cost=1618 Card
          =297964 Bytes=53633520)
   3    1     SORT (AGGREGATE)
   4    3       VIEW OF 'PAYROLL_VW1' (VIEW) (Cost=2290 Card=2 Bytes=
          46)
   5    4         SORT (UNIQUE) (Cost=2290 Card=2 Bytes=217)
   6    5           UNION-ALL
   7    6             FILTER
   8    7               TABLE ACCESS (FULL) OF 'PAYROLL2012' (TABLE) (C
          ost=2284 Card=1 Bytes=105)
   9    6             FILTER
  10    9               TABLE ACCESS (BY INDEX ROWID) OF 'PAYROLL2011'
          (TABLE) (Cost=4 Card=1 Bytes=112)
  11   10                 INDEX (RANGE SCAN) OF 'PAYROLL2011_IDX1' (IND
          EX) (Cost=3 Card=29)So what do I do with the execution plan? What else do I need to check on it? How do I tune the query?
Edited by: apex_disco on May 1, 2012 5:57 AM

Answer 1

I'm not sure how the licensing works. There is a difference between EM and Grid Control. I believe if you just have a single instance, Enterprise Manager is installed and if anything is free I would think that would be. Now, how much of the diagnostic stuff is free? Who knows.
But it may not matter. If you are just playing around "trying to learn SQL Tuning" say, at home on a Linux box, you can install anything you want and play with it.
If adding an index sped things up (in your re-written query that now makes sense) then you must have had a fair amount of records in one of those unindexed tables. If you look at the explain plan now it'll probably show the indexes being used, less records searched, less cost, etc. than the other plan.
But maybe I missed your initial point. Are you trying to learn how to tune queries in general or are you trying to tune this particular query? I assumed the former for my previous replies.
As to your results, your stats </font>tell at least the high level info. The consistent gets and physical reads went WAY down.
Initial Statistics
0 recursive calls
0 db block gets
<font color="red"> 1172490142 consistent gets
4914 physical reads
191496 redo size</font>
2340 bytes sent via SQL*Net to client
329 bytes received via SQL*Net from client
1 SQL*Net roundtrips to/from client
<font color="red">115881 sorts (memory) </font>
0 sorts (disk)
0 rows processed
on those tables and below are my stats now. I think it's a major improvement. Time elapsed has greatly been reduced to 00:00:00.65
Tuned Statistics
352 recursive calls
0 db block gets
<font color="red">31498 consistent gets
1101 physical reads
0 redo size
</font>
2340 bytes sent via SQL*Net to client
329 bytes received via SQL*Net from client
1 SQL*Net roundtrips to/from client
<font color="red">13 sorts (memory) </font>
0 sorts (disk)
0 rows processed
Edited by: Gaff on May 1, 2012 1:18 PM