Domain indexes are used to speed up data retrieval for specific domains like text, spatial or image data. Oracle has built in indexes for above mentioned domains but developers can create their own using extensible indexing interface (ODCIIndex).
Let’s consider specific task when this can be useful and discover process end-to-end.
Frankly speaking “evoulution” is not quite proper word to title this post.
I’m going to show some extensions in regular expressions in different languages in compare to their implementation in Oracle.
Oracle supports regular expressions according to POSIX standard draft 1003.2/D11.2 and two extensions
Let’s consider quizzes which cannot be easily resolved using built-in capabilities in Oracle.
Continue reading “Evolution of regular expressions”
There are quite a few tools and ways to build and manage cubes in Oracle so in this post I’ll try to clarify some concepts.
Good top level introduction could be found in article by Arup Nanda – Data Warehousing and OLAP.
Detailed step by step tutorial – Building OLAP Cubes.
The easiest way to start is to explore existing sample schema.
Continue reading “Oracle OLAP option. Quick dive.”
Let’s consider “hypothetical” case when it’s required to check if row is blocked without attempting to lock it.
And maybe we’ll find a way to see uncommitted data in Oracle. 🙂
Continue reading “Is a row locked?”
New syntax for so called “Top N” queries was introduced in Oracle 12c.
It can be used to return either fixed number of rows or specified percent of rows in a table.
Obviously latter requires to know the total number of rows to return the result so let’s check what is happening under the hood in this case.
Continue reading “fetch first … percent”
I’ve described how to measure context switches some time ago in this post.
Given that with_clause allows to declare PL/SQL functions in 12c it’s interesting to analyze overhead for context switches in this case.
Continue reading “Context switches may not be an issue any longer”
Bloom filter may help to dramatically reduce amount of data to be read from partitioned table by eliminating unneeded partitions.
Here is a nice introduction into bloom filters.
Unfortunately bloom filter doesn’t work with DML which causes a lot of challenges in 11gR2.
This bug is documented on metalink Bug 13801198 : BLOOM PRUNING/FILTER NOT USED IN DML STATEMENTS however that note doesn’t suggest any workarounds.
Continue reading “Yet another approach to overcome bloom filter bug”
Almost everyone who has some experience with Oracle knows about notorious notion called “context switches” between SQL and PL/SQL engines.
You can find a great explanation of context switches by Tom Kyte here.
Even though it’s quite simple to measure the overhead introduced by context switches, not so many developers know how to calculate exact number of context switches and moreover how to check whether they occurred or not in some cases.
Continue reading “Measuring context switches”
dbms_comparison was introduced in 11g as a powerful tool to compare and synchronize table data in different tables.
Even though it’s quite flexible it has some limitations:
1. The tables are supposed to be in different databases. Therefore if you want to compare tables in the same database you will need to create fake db link.
2. The tables must have unique indexes.
Another consequence from the flexibility is that synchronizing is being applied in two steps (merge & delete) even though it could have been done using single merge statement.
Moreover those statements are based not only on tables that are synchronized but also on auxiliary data populated after comparison.
I wonder why Oracle has not added possibility to change the list of statistics for autotrace but unfortunately that’s a fact.
You cannot change the default format of the statistics report.
However there are at least three different ways to view the extended list of statistics for given session
First drawback is that enabling and disabling of stats collecting must be executed in another session and second drawback is that it collects only 27 statistics.
Advantage is the ease of monitoring several sessions with the same client_id.
The disadvantage is the need to run service queries in second session to avoid side affects and positive aspect is that all existed statistics can be reported.
It’s not necessary to create another session manually though. It’s possible to implicitly execute some statements in another session, for example by using dbms_job/dbms_scheduler.
The drawbacks are the same as for previous approach: either you have to run runstats in another session or results are not absolutely precise.
Even though documentation says that we cannot change the default format of the statistics report, it’s in fact possible to amend it.
The only restriction is that number of statistics cannot exceed 11.
Continue reading “Amending statistics list for autotrace”
SQLMDX 