OIC(A) again – 2
Continuing from the previous post, here is one more case when adjusting optimizer_index_cost_adj may hurt you. Setup: drop table t1 cascade constraints purge; create table t1 (id, x, pad, constraint...
View ArticleBloom Filter
I’ve posted this note as a quick way of passing on an example prompted by a twitter conversation with Timur and Maria about Bloom filters:@tmmdv@SQLMaria Also have one that looks like filtering an...
View ArticleNVL()
Here’s a little detail that I hadn’t noticed before (and it goes back to at least 8i). This is running on 11.2.0.3, and table t1is just all_objects where rownum <= 20000:read more
View ArticleMV Refresh
Here’s a funny little problem I came across some time ago when setting up some materialized views. I have two tables, orders and order_lines, and I’ve set up materialized view logs for them that allow...
View ArticleHash Joins
I’ve written notes about the different joins in the past – but such things are always worth revisiting, so here’s an accumulated bundle of comments about hash joins.read more
View ArticleExtended Stats
Here’s a little demo cut-n-pasted from a session running Oracle 12.1.0.1 (it works on 11g, too). All it does is create a table by copying from a well-known table, gather extended stats on a column...
View ArticleVirtual Stats
Or – to be more accurate – real statistics on a virtual column.This is one of the “10 top tips” that I came up with for my session with Maria Colgan at OOW13. A method of giving more information that...
View ArticleFBI decode
It probably won’t surprise many people to hear me say that the decode() function can be a bit of a nuisance; and I’ll bet that quite a lot of people have had trouble occasionally trying to get...
View ArticleView Data Volume Estimates
When the optimizer has to estimate the data volume (the BYTES column in the plan output), it usually bases this information on the column statistics, if applicable and available (think of complex...
View ArticleOutline hassle
Here’s the output I got from a 10.2.0.5 system after generating a stored outline on a query – then dropping the index that was referenced by the stored outline and creating an alternative index. Spot...
View Article12c Subqueries
When you upgrade you often find that some little detail (of the optimizer) that didn’t receive a lot of attention in the “New Features” manuals introduces a few dramatic changes in execution plans....
View ArticleIndex Hash
I’m afraid this is one of my bad puns again – an example of the optimizer making a real hash of the index hash join. I’m going to create a table with several indexes (some of them rather similar to...
View ArticlePredicate Order
Common internet question: does the order of predicates in the where clause make a difference. General answer: It shouldn’t, but sometimes it will thanks to defects in the optimizer.There’s a nicely...
View ArticleEmpty Hash
A little while ago I highlighted a special case with the MINUS operator (that one of the commentators extended to include the INTERSECT operator) relating to the way the second subquery would take...
View ArticleSubquery Anomaly
Here’s an oddity that appeared on the OTN database forum last night:We have this query in our application which works fine in 9i but fails in 11gR2 (on Exadata) giving an “ORA-00937: not a single-group...
View ArticleNVL() change
One of the problems of functions is that the optimizer generally doesn’t have any idea on how a predicate based on function(col) might affect the cardinality. However, the optimizer group are...
View ArticleNVL() change
One of the problems of functions is that the optimizer generally doesn’t have any idea on how a predicate based on function(col) might affect the cardinality. However, the optimizer group are...
View ArticleCosting Bug
It’s amazing how you can find little bugs (or anomalies) as soon as you start to look closely at how things work in Oracle. I started to write an article for All Things Oracle last night about...
View ArticleFeature Bypass
Here’s a little tip that might be helpful occasionally when you’re trying to work out why the optimizer transformation you were expecting isn’t appearingIf you’ve ever checked the 10053 trace (and who...
View ArticleSubquery with OR
Prompted by a pingback on this post, followed in very short order by a related question (with a most gratifying result) on Oracle-L, I decided to write up a note about another little optimizer...
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