Just like my posting on an index hash, this posting is about a problem as well as being about a hash join. The article has its roots in a question posted on the OTN database forum, where a user has shown us the following execution plan:
A recent posting on the comp.databases.oracle.server newsgroup pointed me to a rather elderly Ask Tom question (originally posed in July 2004, last updated June 2011) where Tom produced an extraordinary observation. The response times for the following two queries are completely different (on Oracle 9.2 on his data set):
I see that Christian Antognini posted a note about an interesting little defect in Enterprise Manager a little while ago - it doesn’t always know how to interpret execution plans. The problem appears in Christians’ example when a filter subquery predicate is applied during an index range scan – it’s a topic I wrote about a few months ago with the title “filter bug” because the plan shows (or, rather, fails to show) a “missing” filter operation, which has been subsumed into the predicate section of the thing that would otherwise have been the first child of the filter operation – the rule of recursive descent through the plan breaks, and the ordering that OEM gives for the operations goes wrong.
My note on “NOT IN” subqueries is one of the most popular on my blog, staying in the top 5 hits for the last five years – but it’s getting a bit old, so it’s about time I said something new about “NOT IN” – especially since the Null Aware Anti Join has been around such a long time. The example I want to talk about is, as so often, something that came up as a problem on a customer site. Here’s a bit of SQL to model the situation, which is currently running under Oracle 184.108.40.206:
Here’s an interesting little conundrum about subquery factoring that hasn’t changed in the recent (220.127.116.11) patch for subquery factoring. It came to me from Jared Still (a fellow member of Oak Table Network) shortly after I’d made some comments about the patch. It’s an example based on the scott/tiger schema – which I’ve extracted from the script $ORACLE_HOME/rdbms/admin/utlsampl.sql (though the relevant scripts may be demobld.sql or scott.sql, depending on version).
As usual I’ve used an 8KB block size, LMT with uniform 1MB extents, and no ASSM to hold the data. I won’t reproduce the code to generate the schema, just the versions of the query with, then without, subquery factoring:
From time to time a request for a query to “show the current SQL” for each session appears on the internet. It’s not really difficult to write something suitable, but this particular request is an interesting demonstration of how much inertia there is in the Oracle world. Here’s a recent suggestion of what the query should look like:
select sess.sid, sess.serial#, sess.username, sess.status, sess.osuser, sess.machine, sess.module, sess.event, sess.logon_time, ss.sql_id, ss.sql_text from v$session sess, v$sql ss where sess.sql_id = ss.sql_id and sess.username = 'EFTUSER' ;
The query has an error in it that appears extremely frequently in response to this request. Can you spot what it is ? (On the plus side, the query references v$sql rather than v$sqlarea – so it’s not too nasty.)
It’s always worth browsing through the list of Oracle’s bug fixes each time a new release or patch comes out because it can give you clues about where to look for problems in your production release – and how to anticipate problems on the upgrade. This article is an example of a fix that I found while while looking at the note for 18.104.22.168 quite recently.
A few days ago I published an example of the optimizer failing to handle an updateable join view because it didn’t recognise that a particular type of aggregate subquery would guarantee key-preservation. Here’s another example where the human eye can see key-preservation, but the optimizer can’t (even in 22.214.171.124). As usual we’ll start with some sample data – in this case two tables since I want to update from one table to the other.
create table t1 ( id1 number, id2 number, val number, constraint t1_pk primary key (id1, id2) ); insert into t1 values (1,1,99); commit; create table t2 ( id1 number, id2 number, id3 number, val number, constraint t2_pk primary key (id1, id2, id3) ); insert into t2 values (1,1,1,200); insert into t2 values (1,1,2,200); commit;
Here’s a funny little optimizer bug – though one that seems to have been fixed by at least 10.2.0.3. It showed up earlier on today in a thread on the OTN database forum. We’ll start (in 126.96.36.199) with a little table and two indexes – one normal, the other descending.
Here’s a simple update statement that identifies a few rows in a table then updates a column where a matching value can be derived from another table – it’s an example of an update by correlated subquery:
update t1 set small_vc = ( select max(small_vc) from t2 where t2.id = t1.id ) where mod(id,100) = 0 and exists ( select null from t2 where t2.id = t1.id ) ;