Comments on: Virtual columns revisited

By: Virtual Columns « Oracle Scratchpad

Virtual Columns « Oracle Scratchpad — Thu, 16 Apr 2009 19:26:32 +0000

[...] **1: In fact, you can collect statistics on “virtual columns” pre-11g by creating function-based indexes in an unusable state. The method works, but it’s not really a good [...]

By: Shakespeare

Shakespeare — Thu, 30 Aug 2007 15:32:14 +0000

Indeed, virtual columns are in 11g now!

By: Mirjana

Mirjana — Fri, 04 May 2007 07:57:58 +0000

Excellent!!!
(Few months too late… but just what I need right now :-)

By: Connor

Connor — Sat, 04 Nov 2006 02:35:09 +0000

Can’t remember which version it came into, but you can also directly reference hidden column names, eg “for column SYS_… size 10″ etc

By: Jeff Moss

Jeff Moss — Fri, 03 Nov 2006 07:53:27 +0000

…I knew about method_opt for getting the histograms – I’ve worked out now that when I was looking for the histogram I was using a little script of mine to give me lots of optimiser stats for the table in question…unfortunately, as I alluded to in the last post, I found that my script happened to be using the DBA_TAB_COLUMNS view rather than the DBA_TAB_COLS view…which meant I couldn’t actually see the virtual column at all let alone whether there was a histogram on it…after fixing my script and retesting all this I find it’s exactly as you say, the histogram can be created and it does help with getting the correct cardinality for a query with a predicate on a column with skewed data in it.

Now our problem is that the value we are testing for BETWEEN the from_date AND NVL(to_date,TO_DATE(’31-DEC-9999′,’DD-MON-YYYY’)) comes from a scalar subquery select…so the optimiser doesn’t know what value it is and can’t get the right cardinality – with a literal date value it works fine…more of an architectural approach issue now I guess.

By: Jonathan Lewis

Jonathan Lewis — Thu, 02 Nov 2006 16:11:52 +0000

Jeff, There is a method_opt for gather_table_stats which does the job. Try 'for all hidden columns size NNN'. This is available from 9.2 if not earlier.

By: Jeff Moss

Jeff Moss — Thu, 02 Nov 2006 15:02:53 +0000

I’m playing with it…but our scenario is a bit different…no time to explain in depth, but our functional column involves an NVL on a NULLable column to the max date of 31-DEC-9999(“Bad Defaults Affect Cardinality” is the slide you present I believe) – our problem is that we can get the virtual column index but can’t get a histogram on it so can’t see the skew we have on the column in order to assist the CBO to get the cardinalty right.

One thing I have just seen though, is that you obtain the column low/high values from USER_TAB_COLS. I tried doing the same select, not realising I’d mistakenly typed USER_TAB_COLUMNS and I couldn’t see the SYS virtual column…on realising my typo and switching back to USER_TAB_COLS it appears…which I thought was an interesting little gotcha.