Saturday, October 6, 2012

XPath Query Rewrite

In this entry I want  to share my experience of tracing down an interesting problem arising from upgrading to Oracle 11g.
After a few days post migration to Oracle of a database I happened to check the alert log and found there the following:

DDE: Problem Key 'ORA 4030' was completely flood controlled (0x6)
Further messages for this problem key will be suppressed for up to 10 minutes

ORA-4030 happens when a server process fails to allocate a portion of PGA memory.
I checked the application and the ETL jobs but nothing seemed to be failing. Therefore I decided to check the PGA memory used by different sessions:

  FROM v$process p,
       v$session s
 WHERE p.addr = s.paddr
 ORDER BY p.pga_alloc_mem DESC;

The first line I saw had had 4293618712 bytes in the PGA_ALLOC_MEM column. That process allocated 4Gb of PGA memory which is the maximum it could get out of Linux. Using the session information I isolated the code causing the issue. Turned out it was a massive (500+ lines) SQL statement heavily utilising XMLElement and XMLForest functions. 

A quick check on metalink revealed a bug #13477790 matched our situation (and later 
confirmed by the Oracle support to be the culprit). It said the problem was exactly due to usage of XML functions. What I couldn't understand from the description is how Oracle managed to get broken the code which was working perfectly on 10g.

So I decided to reproduce the problem. I isolated the code and when I ran it, I noticed that Oracle does not even start returning the rows from the query that was failing. Ergo I decided to trace CBO.

What I found in the 10053 trace file was lots of attempts to transform the query and then dies with ORA-4030.
In parallel I was googling what has changed in 11g in terms of XDB, and overall information about XDB (as I hasn't worked with it), and found a hint NO_XML_QUERY_REWRITE. I googled it and found that XDB has got a thing called XPath rewrite, when Oracle rewrites a query to optimise XML query execution. In my case no rewriting could be useful, as we don't use XDB per se - we just construct XML from plain tables and upload it as clob. So I tried it, and it helped. 

Eventually we applied a patch to fix the problem permanently.

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