This is possibly my longest title to date – I try to keep them short enough to fit the right hand column of the blog without wrapping – but I couldn’t think of a good way to shorten it (Personally I prefer to use the expression CTE – common table expression – over “factored subquery” or “subquery factoring” or “with subquery”, and that would have achieved my goal, but might not have meant anything to most people.)
If you haven’t come across them before, recursive CTEs appeared in 11.2, are in the ANSI standard, and are (probably) viewed by Oracle as the strategic replacement for “connect by” queries. Here, to get things started, is a simple (and silly) example:
From time to time I’ve posted a reminder that subquery factoring (“with subquery”) can give you changes in execution plans even if the subquery that you’ve taken out of line is written back inline by Oracle rather than being materialized. This can still happen in 12c – here’s a sample query in the two forms with the result sets and execution plans. First, the “factored” version:
I’ve written a few notes about anomalies in subquery factoring (with subquery) in the past, principally making a fuss about the fact that moving an inline view into a “with subquery” can cause a plan to change even when the internal code moves the subquery back in line. With the arrival of 12c one of my first sets of tests was to rerun all the examples to see how many of them had been addressed. I hadn’t written about as many examples as I had thought, and some of them had been fixed before 12c, but here are few references to a couple of outstanding items that I thought worth a mention:
[Further reading on "subquery factoring"]
I have a small collection of postings where I’ve described anomalies or limitations in subquery factoring (the “with subquery”, or Common Table Expression (CTE) to give it the official ANSI name). Here’s another example of Oracle’s code not behaving consistently. You may recognise the basic query from yesterday’s example of logical tuning – so I won’t reprint the code to generate the data sets. This examples in this note were created on 184.108.40.206 – we start with a simple query and its execution plan:
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).
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 (MOS licence required for link) quite recently.
The UKOUG conference is over for another year – but it has left me with plenty to do and lots of things to investigate. Here’s just one little point that I picked up during one of the 10 minute “Oak Talks” that members of the Oak Table Network were doing in the lunch breaks.
There is a fairly well-known strategy for generating a list of numbers by using a “select from dual … connect by …” query, but I hadn’t realised that there were two ways of using it. The code I’ve usually used is this:
rownum <= 4000
I’ve written before about the effects of subquery factoring (common table expressions – or CTEs) on the optimizer, and the way that the optimizer can “lose” some strategies when you start factoring out subquery expressions. Here’s another example I came across quite recently. It involved a join of about 15 tables so I’ve only extracted a few lines from the SQL and resulting execution plans.
We start with the original query, which had factored out an aggregate subquery then used it in place of an inline view:
From time to time I’ve warned people that subquery factoring should be used with a little care if all you’re trying to do is make a query more readable by extracting parts of the SQL into “factored subqueries” (or Common Table Expressions – CTEs – if you want to use the ANSI term for them). In principle, for example, the following two queries should produce the same execution plan:
I’ve written about subquery factoring a few times in the past and commented on the use of the /*+ materialize */ hint. Recently I had time to think about what it would take for the Cost Based Optimizer to decide to materialize a subquery without hinting.
I doubt if I have a complete answer yet, and I sometimes wonder if the optimizer code for handling subquery factoring is not yet complete, but my observations are as follows.
A recent newsgroup question asked whether or not the “with subquery” clause – introduced as part of the select statement in 9i – could also be used with the merge statement. The answer is yes, provided you remember that the merge statement includes a select, and the subquery belongs to the select. The following, for example, is valid syntax:
into old_data od
with m_subq as(
where mod(id,50) = 0
select * from m_subq
od.id = nd.id
and od.small_vc = nd.small_vc
when matched then
update set od.padding = nd.padding
when not matched then
insert values ( nd.id, nd.small_vc, nd.padding )
Remember, before you get too excited by this option, that if the subquery is materialized, Oracle will create an “in memory metadata” global temporary table and write the result of the subquery to your temporary tablespace using direct path writes before reading it back with db file scattered reads.
[Further reading on "subquery factoring"]