Interval Oddity
Interval partitioning is a popular strategy for partitioning date-based data. It’s an enhanced variant of range partitioning that allows you to define a starting partition and an interval that should...
View ArticleInline Hint
If you’ve ever used subquery factoring (“with” subqueries or common table expressions (CTEs) as they are often called) then you’re probably aware of the (undocumented) hints /*+ materialize */ , which...
View ArticleIndex FFS Cost
There are a number of unexpected issues with the optimizer’s treatment of the index fast full scan, the access path where Oracle ignores the structure of the B-tree and uses multiblock reads to do a...
View ArticleSerial Bloom
Following the recent note I wrote about an enhancement to the optimizer’s use of Bloom filters, I received a question by email asking about the use of Bloom filters in serial execution plans:I’m having...
View ArticleSubquery with OR
I’ve written a coupleof notesin the past about the problems of optimising queries with predicates of the form “or exists {subquery}”. A recent question on the Oracle Developer Community forum brought...
View ArticleFetch First vs. Rownum
I’ve pointed out fairly frequently that if you’re running Standard Edition but would like to take advantage of a few features of the Partitioning option then you might be able to do something...
View ArticleRecursive WITH upgrade
There’s a notable change in the way the optimizer does cost and cardinality calculations for recursive subquery factoring that may make some of your execution plans change – with a massive impact on...
View ArticleExecution Plans
This is an example from the Oracle Developer Community of using the output of SQL Monitor to detect a problem with object statistics that resulted in an extremely poor choice of execution plan.A short...
View ArticleMost Recent – 2
A question arrived in my email a few days ago with the following observations on a statement that was supposed to query the data dictionary for some information about a specified composite partitioned...
View ArticleExecution Plans
In previous articles on reading execution plans I’ve made the point that the optimizer is very “keen” to transform complex queries into queries consisting of a single query block and that there’s a...
View ArticleAnalytic cost error
Here’s a surprising costing error that was raised on the Oracle Developer Forum a few days ago. There’s a glitch in the cost atributed to sorting when an analytic over() clause – with corresponding...
View Articlefrom$_subquery$_NNN
This is a reference note for a question that came up as a comment on a lengthy note I wrote about reading execution plans.How do you interpret something like: from$_subquery$_001@SEL$1 in the Query...
View ArticleExecution Plans
In a comment to a recent post on reading a non-trivial execution someone asked me to repeat the exercise using a plan I had published a few days previously in a post about tweaking the hints in an...
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 (hereand...
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 ArticleExecution Plans
In a recent blog note I made the point that there is a very simple rule (“first child first”) for reading execution plans if the query (as written or after transformation by the optimizer) consists of...
View Articledate_to_date
Every now and again someone posts a piece of SQL on the Oracle Developer Forum that includes a predicate with an expression like to_date(date_column). This is a problem for several reasons – not the...
View ArticleEureka!
I woke up last night with a brilliant solution to a problem that’s been bugging me for more than a year. How does a call to report_sql_monitor() manage to produce output like this:read more
View ArticleExecution Plans
One of the most important skills needed when investigating badly performing SQL is the ability to read Execution Plans. It’s a topic I’ve written and spoken about frequently – even to the extent of...
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...
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