v$sql_hint
Here’s a quick and dirty script I thought I’d published a very long time ago. It has to be run as SYS and queries v$sql_hint to produce a list of all the things which (at least in theory) could be...
View ArticleAdaptive Joins
There’s a question on the Oracle Forums at the moment about a query that’s taking a long time to parse. Even after being reduced to one prebuilt (currently remote) table with two non-correlated outer...
View ArticleHinting
This is just a lightweight note on the risks of hinting (which might also apply occasionally to SQL Plan Baselines). I’ve just rediscovered a little script I wrote (or possibly last tested/edited) in...
View Articleopt_estimate 4a
I wrote a batch of notes about the opt_estimate() hint a couple of years ago, including one where I explained the option for using the hint to specify the number of rows in a query block. I’ve just...
View ArticleCase Study
A recent post on the Oracle SQL and PL/SQL forum posted the following query with a complaint that it produced the wrong results: select count(*) from all_synonyms left join all_objects b on...
View Articleno_merge() #JoelKallmanday
This is a second follow-up to the video Connor McDonald produced a few days ago about the risks of depending on “current tricks” to control the order of predicate operation, then showing how some...
View ArticlePush Group by
Jump to summary. A new optimizer feature that appears in 23c (probably not 21c) was the ability to push group by clauses into union all set operations. This will happen unhinted, but can be hinted...
View ArticleDescending max()
I’ve written a few notes about problems with “descending” indexes in the past (the word is in quotes because it’s not the index that’s defined as descending, it’s a proper subset of the columns of the...
View ArticleSwap_Join_Inputs
This is just a short note (I hope) prompted by a conversation on the Oracle-L list server. A query from a 3rd party application was doing a very large full tablescan with hash join when it should have...
View Articlegby_pushdown
This is a note that’s been awaiting completion for nearly 10 years. It’s about a feature (or, at least, a hint for the feature) that appeared in 10.2.0.5 to control some of the inter-process messaging...
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