Wednesday, October 10, 2012

SQL Loader's Append

I've recently bumped into one peculiarity of SQLLoader, particularly its APPEND mode. SQLLoader's LOAD APPEND works in the same way as DML INSERT /*+ APPEND*/, that is makes Oracle to allocate new blocks above HWM, right? Well, not quite.
Here's a little demo on 10g R2 on Solaris:
SQL> create table t1
  2  (
  3    id  number(12),
  4    padding varchar2(255)
  5  );

Table created.

  2  INTO t1
  3    SELECT rownum,
  4           rpad('x', 255)
  5      FROM dual
  6    CONNECT BY LEVEL <= 1000;

1000 rows created.

SQL> commit;

Commit complete.

SQL> delete from t1;

1000 rows deleted.

SQL> commit;

Commit complete.

SQL> set serveroutput on
  2    free_blks NUMBER;
  3  BEGIN
  4    dbms_space.free_blocks(USER, 'T1', 'TABLE', 0, free_blks);
  5    dbms_output.put_line(free_blks);
  6  END;
  7  /

PL/SQL procedure successfully completed.
So far I've created a small table, populated it with a thousand rows, then deleted all the data and committed the transaction. Then I checked the number of free blocks in the freelist - it is 19.

Now let's load some data via SQL Loader using a control file as follows:
OPTIONS(readsize=20000000, bindsize=20000000, rows=8000)
FIELDS TERMINATED BY "|" optionally enclosed by '"' TRAILING NULLCOLS
1|"x     <<254 whitespaces="whitespaces">>"
I've removed the data section to save space, but you've got the idea. Now let's run it:
SQL> host sqlldr userid=user/pwd@db control=ldr.ctl log=ldr.log

  2    free_blks NUMBER;
  3  BEGIN
  4    dbms_space.free_blocks(USER, 'T1', 'TABLE', 0, free_blks);
  5    dbms_output.put_line(free_blks);
  6  END;
  7  /

PL/SQL procedure successfully completed.

  2    dbms_stats.gather_table_stats(ownname => USER, tabname => 'T1',
  3                                  cascade => TRUE,
  4                                  method_opt => 'for all columns size 1');
  5  END;
  6  /

PL/SQL procedure successfully completed.

SQL> select blocks from user_tab_statistics where table_name = 'T1';

As you can see, we still have got the same 19 blocks in the table and only 1 of them is listed as free in the free list.

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.