pushing predicates
I came across this odd limitation (maybe defect) with pushing predicates (join predicate push down) a few years ago that made a dramatic difference to a client query when fixed but managed to hide...
View ArticleSubquery Order
From time to time I’ve wanted to optimize a query by forcing Oracle to execute existence (or non-existence) subqueries in the correct order because I know which subquery will eliminate most data most...
View Articlenum_index_keys
The title is the name of an Oracle hint that came into existence in Oracle 10.2.0.3 and made an appearance recently in a question on the rarely used “My Oracle Support” Community forum (you’ll need a...
View ArticleHint 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...
View ArticleAppend hint
One of the questions that came up on the CBO Panel Session at the UKOUG Tech2018 conference was about the /*+ append */ hint – specifically how to make sure it was ignored when it came from a 3rd party...
View ArticleDanger – Hints
It shouldn’t be possible to get the wrong results by using a hint – but hints are dangerous and the threat may be there if you don’t know exactly what a hint is supposed to do (and don’t check very...
View ArticleIgnoring Hints
One of the small changes (and, potentially big but temporary, threats) in 18.3 is the status of the “ignore hints” parameter. It ceases to be a hidden (underscore) parameter so you can now officially...
View Articleopt_estimate 2
This is a note that was supposed to be a follow-up to an initial example of using the opt_estimate() hint to manipulate the optimizer’s statistical understanding of how much data it would access and...
View Articleopt_estimate 3
This is just a quick note to throw out a couple of of the lesser-known options for the opt_estimate() hint – and they may be variants that are likely to be most useful since they address a problem...
View Articleopt_estimate 4
In the previous article in this series on the opt_estimate() hint I mentioned the “query_block” option for the hint. If you can identify a specify query block that becomes an “outline_leaf” in an...
View Articleopt_estimate 5
If you’ve been wondering why I resurrected my drafts on the opt_estimate() hint, a few weeks ago I received an email containing an example of a query where a couple of opt_estimate() hints were simply...
View Articleopt_estimate catalogue
This is just a list of the notes I’ve written about the opt_estimate() hint. opt_estimate – using the hint to affect index calculations: index_scan and index_filter opt_estimate 2 – applying the hint...
View ArticleANSI Plans
Here’s a thought that falls somewhere between philosophical and pragmatic. It came up while I was playing around with a problem from the Oracle database forum that was asking about options for...
View Articlepush_having_to_gby() – 2
The problem with finding something new and fiddling with it and checking to see how you can best use it to advantage is that you sometimes manage to “break” it very quickly. In yesterday’s blog note I...
View ArticleClik here to view.
Collection limitation
The ODC SQL and PL/SQL forum came up with an example a couple of days ago that highlighted an annoying limitation in the optimizer’s handling of table functions. The requirement was for a piece of SQL...
View ArticleFake Baselines – 2
Many years ago (2011) I wrote a note describing how you could attach the Outline Information from one query to the SQL_ID of another query using the official Oracle mechanism of calling...
View ArticleANSI hinting
I’ve made casual remarks in the past about how “ANSI”-style SQL introduces extra complications in labelling or identifying query blocks – which means it’s harder to hint correctly. This is a note to...
View ArticleExecution Plans
A couple of days ago I discussed an execution plan that displayed some variation in the way it handled subqueries and even threw in a little deception by displaying an anti-join that was the result of...
View ArticleHint hacking
How do you work out what hints you need to tweak an execution plan into the shape you want? Here’s a “case study” that’s been playing out over a few weeks on the Oracle Developer Community (here and...
View ArticleFetch First Update
A question about mixing the (relatively new) “fetch first” syntax with “select for update” appeared a few days ago on the Oracle Developer Forum. The requirement was for a query something like: select...
View Article