If you have been involved in tuning SQL code which you have never seen before, you are probably familiar with the challenges of understanding what the code is trying to do. This can be especially time consuming when the SQL references lots of views, which reference views, which reference more views etc. So there may be a large information gap between the SQL statement (like select * from some_crazy_10_level_view) and the actual execution plan (referencing 10s of tables, with evidence of query transformations).
So unless you see something really obvious from the execution plan, you need to start mapping the SQL query and view texts back to the physical base tables which Oracle eventually has to access. This can be a tedious and boring (!) process.
The good news is that in Oracle 10.2+ there’s a hidden parameter that can do this mapping task for us.
Update: It looks like 18.104.22.168 patchset also has this parameter (so I guess later patchsets of 10.1.0.x have it as well now)
Let’s see an example:
I create a view on a view to illustrate the point:
SQL> create view myview as select * from all_users; View created.
Now let’s set that parameter _dump_qbc_tree to 1 and run a query against the view:
SQL> alter session set "_dump_qbc_tree"=1; Session altered. SQL> select count(*) from myview; COUNT(*) ---------- 31
Now let’s look into the server process tracefile:
*** ACTION NAME:() 2007-09-16 12:19:57.500 *** MODULE NAME:(SQL*Plus) 2007-09-16 12:19:57.500 *** SERVICE NAME:(SYS$USERS) 2007-09-16 12:19:57.500 *** SESSION ID:(146.1984) 2007-09-16 12:19:57.500 QCSDMP: ------------------------------------------------------- QCSDMP: SELECT: (qbc=2B8D1C28) QCSDMP: . (COUNT(*)) (opntyp=2 opndty=0) QCSDMP: FROM: QCSDMP: .MYVIEW QCSDMP: VQB: QCSDMP: SELECT: (qbc=2B8D163C) QCSDMP: .USERNAME QCSDMP: FROM: QCSDMP: .ALL_USERS QCSDMP: VQB: QCSDMP: SELECT: (qbc=2B8CAF78) QCSDMP: U.NAME (USERNAME) QCSDMP: FROM: QCSDMP: SYS.TS$ (TTS) QCSDMP: SYS.TS$ (DTS) QCSDMP: SYS.USER$ (U)
Here it is, the query text generated directly from parse tree, showing the base tables regardless that they had been hidden behind multiple views.
Also there’s few interesting things to note: