ALL POSTS

Multi-Column Correlation and Extended Stats in Oracle 11g

We all have encountered this situation many times before: the cost-based optimizer assumes no correlation between two columns (until 11g), and this has the effect of erroneously reducing the cardinality of a row source. Incorrect cardinality estimates are one of many root causes for SQL performance issues. Consider the following example.

Another Tip on Using dg4odbc: Materialized Views

After setting up dg4odbc to connect to a SQL Server database (read ‘Tips on Using dg4odbc on 64-bit Linux’), I spent last week trying to get refreshes of materialized views to work in 11g, using the same DDL as in 8i. So, to those three tips, I now add #4: check the statements that use a WHERE … IS NOT NULL across the remote database link to ANSI-compliant databases.

Oracle ASM 11g: Does the ASMCMD cp Command Really Work?

Since the introduction of ASM in Oracle 10g Release 1, every ASM administrator has been dreaming of a simple command line tool to copy files between ASM diskgroups and other filesystems. Oracle ASM 10g Release 2 added the handy asmcmd utility, but even though everyone expected a copy command there, it had not been implemented. While working on a Collaborate 08 presentation on Oracle 11g new features out-of-the-box, I was verifying new commands in Oracle ASM 11g’s asmcmd utility. It turned our that copying files from or to ASM is still a problem.

How to Make an In-Database listener.log File

Ever wished the listener.log file was a table in the database? Wish no more! About three years ago, I sent this recipe in an email to my co-workers. Just recently, Shakir re-sent it after using the method in an emergency. Since it seems to have proved its value, I now offer it to our readers. Have a look.

When SHOW SLAVE STATUS lies

Over-the-Top Tales from the Trenches: Bringing order to the chaos of every day DBA life – So you have got your nice MySQL Master-Slave replication pair setup. Everything is sweet, then the master dies/restarts or you have a slightly extended network outage. You log into the machine and check out why the slave threw an error or if your monitoring is slow, why the slave is lagging by 2 hours. You run SHOW SLAVE STATUS\G

Bad SQL or MySQL Bug?

One of my colleagues made a typo in a query today that led to me discovering this issue — I think it’s a bug, though it may just be how the language is defined. But certainly a subquery that cannot run should not act as if it returns TRUE. Part of me thinks there’s an implicit join going on or something, but I’m not sure how that’s working. Have a look.

How To Identify the Database Links of a Failed Refresh Job

I have a client with refreshes scheduled using MV refresh groups, and it took me a while to recall the view name, asking around and RTFM’ing. Since this situation comes up regularly, I wrapped up a quick script that parses job content, takes in account MV refresh groups, and outputs the database link(s) involved. I think it could be useful to few others so here it goes.

Rajaraman’s First Law: More Data will beat Better Software

After the interesting comment storm on Doug’s blog when he posted some of Tim Gorman’s comments on the value of data in his career experiences as compared to the value of the applications manipulating that data, I hesitate a little to post this. But, I can’t stop myself because it’s such an interesting insight! So here it is!

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