I have a table with 1.5 million records that needs to have a column updated based on a correlated subselect.
update updtbl a set updcol = (select rtnval from srctbl s where s.qcol1=a.qcol1 and s.qcol2=a.qcol2 and s.qcol3=a.qcol3 and s.qcol4=a.qcol4);
All the columns referenced in srctbl, including rtnval, are columns in an index; qcol columns are the first ones in the index.
This update is currently sitting at four hours and counting. The database server is on a Linux box. I have a lot more updates such as this to perform on this database. What can I do to improve this performance?
- Is the updcol column on the updtbl table indexed? If it is, I would first drop or disable the index, and then run this update. After the update is complete, then rebuild the index.
- Have you run an Explain Plan on the entire update statement to verify it is using the index on the srctbl in the subquery? If not, please do so to see what access path the optimizer is using. If it is not using an index it is doing a table scan of the scrtbl table for each row in the updtbl table. Also, look at the subquery and run an explain plan on it as well. How long does the subquery take to run when run by itself? If it is using the index what type of access on the index is it using? If the subquery takes a long time to run, then start by tuning it first.
Dig Deeper on Oracle database performance problems and tuning
Related Q&A from Phillip Bracken
I have always used TKPROF to do SQL tuning in previous versions of Oracle (7,8 and 9). Can I still use TKPROF in Oracle 10g R2?continue reading
A user complains that every time he tries to access a table (select only), it takes more than two hours to get the results. There are no DML ...continue reading
I tried to import a database dump onto my Oracle server using the imp script. The import process currently takes 13 hours for a 3 GB dump file.continue reading
Have a question for an expert?
Please add a title for your question
Get answers from a TechTarget expert on whatever's puzzling you.