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FK on delete

This is part 1 of a short reference note about the work load created by referential integrity constraints when you delete from a parent table. It was prompted by a question on the Oracle Developer Community forum about how to handle a very large delete from a table which (a) included a column of type CLOB and (b) had 9 child tables.

The 9 referential integrity constraints were declared with “on delete cascade”, but the delete was taking too long even though all the related child data had been deleted before the parent delete. In outline the process was designed to operate in batches as follows:

Massive Deletes

One of the recurrent questions on the Oracle Developer Commuity forum is:

What’s the best way to delete millions of rows from a table?

There are an enormous number of relevant details that you need to know before you can give the “right” answer to this question, e.g.

maxquerylen

The view v$undostat is a view holding summary information about undo activity that can be used by the automatic undo mechanism to deal with optimising the undo retention time (hence undo space allocation). The view holds one row for every ten minute interval in the last 4 days (96 hours) and includes two columns called maxquerylen and maxqueryid – which tell you something about the query that was considered to be the longest running query active in the interval.

In this note I want to explain why the contents of these two columns are sometimes (possibly often) completely irrelevant despite there being a few notes on the internet about how you should investigate them to help you decide on a suitable setting for the undo_retention.

WITH Subquery

Here’s another anomaly that appears when you mix and match Oracle features. In this case it’s “With” subqueries (common table expressions / CTEs) and Active Dataguard (ADG) Standby databases. The problem appeared on the Oracle-l listserver and luckily for the OP another member of the list had seen it before and could point to a relevant MOS document id which explained the issue and supplied a workaround.

The OP had their standby database opened read-only for reporting and found the following oddity in the extended SQL trace file for one of their reports:

Nested Tables

This note is a little side effect of answering a question about the total space associated with a table, including its indexes, LOB columns and nested tables. The first suggestion I published failed to take account of the fact that nested tables can contain their own nested tables so I had to modify the code.

IOT Hash

It’s another of my double-entendre titles. The optimizer can turn a hash join involving an index-organized table into a real performance disaster (though you may have to help it along the way by using a silly definition for your primary key columns). This post was inspired by a question posted on the Oracle Developer Community forum recently so the table and column names I’ve used in my model reflect (almost, with a few corrections) the names used in the post.

We start with a simple requirement expressed through the following SQL:

v$session

Here’s an odd, and unpleasant, detail about querying v$session in the “most obvious” way. (And if you were wondering what made me resurrect and complete a draft on “my session id” a couple of days ago, this posting is the reason). Specifically if you want to select some information for your own session from v$session the query you’re likely to use in any recent version of Oracle will probably be of the form:


select {list for columns} from v$session where sid = to_number(sys_context('userenv','sid'));

Unfortunately that one little statement hides two anomalies – which you can see in the execution plan. Here’s a demonstration cut from an SQL*Plus session running under 19.3.0.0:

My SID

Here’s a little note that’s been hanging around as a draft for more than eight years according to the OTN (as it was) posting that prompted me to start writing it. At the time there were still plenty of people using Oracle 10g. so the question didn’t seem entirely inappropriate:

On 10g R2 when I open a sqlplus session how can I know my session SID ? I’m not DBA then can not open as sysdba and query v$session.

Resumable

There are two questions about temporary space that appear fairly regularly on the various Oracle forums. One is of the form:

From time to time my temporary tablespace grows enormously (and has to be shrunk), how do I find what’s making this happen?

The other follows the more basic pattern:

My process sometimes crashes with Oracle error: “ORA-01652: unable to extend temp segment by %n in tablespace %s” how do I stop this happening?

Before moving on to the topic of the blog, it’s worth pointing out two things about the second question:

Trace Files

A recent blog note by Martin Berger about reading trace files in 12.2 poped up in my twitter timeline yesterday and reminded me of a script I wrote a while ago to create a simple view I could query to read the tracefile generated by the current session while the session was still connected. You either have to create the view and a public synonym through the SYS schema, or you have to use the SYS schema to grant select privileges on several dynamic performance views to the user to allow the user to create the view in the user’s schema. For my scratch database I tend to create the view in the SYS schema.

Script to be run by SYS: