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Execution plans

Can’t Unnest

In an echo of a very old “conditional SQL” posting, a recent posting on the ODC general database discussion forum ran into a few classic errors of trouble-shooting. By a lucky coincidence this allowed me to rediscover and publish an old example of parallel execution gone wild before moving on to talk about the fundamental problem exhibited in the latest query.

The ODC thread started with a question along the lines of “why isn’t Oracle using the index I hinted”, with the minor variation that it said “When I hint my SQL with an index hint it runs quickly so I’ve created a profile that applies the hint, but the hint doesn’t get used in production.”

Parallel Fun – 2

I started writing this note in March 2015 with the following introductory comment:

A little while ago I wrote a few notes about a very resource-intensive parallel query. One of the points I made about it was that it was easy to model, and then interesting to run on later versions of Oracle. So today I’m going to treat you to a few of the observations and notes I made after modelling the problem; and here’s the SQL to create the underlying objects:

Scalar Subquery Costing

A question came up on Oracle-l list-server a few days ago about how Oracle calculates costs for a scalar subquery in the select list. The question included an example to explain the point of the question. I’ve reproduced the test below, with the output from an 18.3 test system. The numbers don’t match the numbers produced in the original posting but they are consistent with the general appearance.

Misleading Execution Plan

A couple of weeks ago I published a note about an execution plan which showed the details of a scalar subquery in the wrong place (as far as the typical strategies for interpreting execution plans are concerned). In a footnote to the article I commented that Andy Sayer had produced a simple reproducible example of the anomaly based around the key features of the query supplied in the original posting and had emailed it to me.  With his permission (and with some minor modifications) I’ve reproduced it below:

Execution Plan Puzzle

Here’s an execution plan that’s just been published on the ODC database forum. The plan comes from a call to dbms_xplan.display_cursor() with rowsource execution statistics enabled.

There’s something unusual about the execution statistics that I don’t think I’ve seen before – can anyone else see anything really odd, or (better still) anything which they would expect others to find odd but which they can easily explain.

A couple of hints:

Lost time

Here’s a little puzzle that came up in the ODC database forum yesterday – I’ve got a query that has been captured by SQL Monitor, and it’s taking much longer to run than it should but the monitoring report isn’t telling me what I need to know about the time.

Here’s a little model to demonstrate the problem – I’m going to join a table to itself (the self join isn’t a necessary feature of the demonstration, I’ve just been a bit lazy in preparing data). Here’s a (competely truthful) description of the table:

Cartesian Join

I wrote this note a little over 4 years ago (Jan 2015) but failed to publish it for some reason. I’ve just rediscovered it and it’s got a couple of details that are worth mentioning, so I’ve decided to go ahead and publish it now.

A recent [ed: 4 year old] question on the OTN SQL forum asked for help in “multiplying up” data – producing multiple rows from a single row source. This is something I’ve done fairly often when modelling a problem, for example by generating an orders table and then generating an order_lines table from the orders table, and there are a couple of traps to consider.

Descending Problem

I’ve written in the past about oddities with descending indexes ( here, here, and here, for example) but I’ve just come across a case where I may have to introduce a descending index that really shouldn’t need to exist. As so often happens it’s at the boundary where two Oracle features collide. I have a table that handles data for a large number of customers, who record a reasonable number of transactions per year, and I have a query that displays the most recent transactions for a customer.

Hint Reports

Nigel Bayliss has posted a note about a frequently requested feature that has now appeared in Oracle 19c – a mechanism to help people understand what has happened to their hints.  It’s very easy to use, it’s just another format option to the “display_xxx()” calls in dbms_xplan; so I thought I’d run up a little demonstration (using an example I first generated 18 years and 11 versions ago) to make three points: first, to show the sort of report you get, second to show you that the report may tell you what has happened, but that doesn’t necessarily tell you why it has happened, and third to remind you that you should have stopped using the /*+ ordered */ hint 18 years ago.

I’ve run the following code on livesql:

Transitive Closure

This is a follow-up to a note I wrote nearly 12 years ago, looking at the problems of transitive closure (or absence thereof) from the opposite direction. Transitive closure gives the optimizer one way of generating new predicates from the predicates you supply in your where clause (or, in some cases, your constraints); but it’s a mechanism with some limitations. Consider the following pairs of predicates: