Select Statement Hangs often

Question

Hello ,
I have very strange issue, Any select statement on few tables takes very long time( in fact it hangs) often but the same SQL runs in a second all other times in the same database. Please note that it is a simple select statement without any 'for update' clause.
After recycling the database, the select statement runs faster but this situation comes back after couple of days on the same set of tables.
We are using 9i release 2.
Can you please advise how to go about solving this type of issue.
Thanks.

Answer 1

438628, Robert is correct in that the board would need to see an explain plan to have any chance of narrowing the problem down. If bind variable peeking is involved the plan may not match what Oracle is actually doing to solve the query.
When the problem occurs try looking in v$plan_table and see if the plan matches the explain plan output. A difference would be a clue that a reparse of sometype is being done so the plan could be different for each execution. The presence of child cursors would also point to something causing reparses.
Do any of the columns for this table involved in the where clause has histograms on them? What is the value of the spfile/init.ora parameter cursor_sharing?
If cursor_sharing is EXACT then bind variable peeking should not be the problem. If the parameter is SIMILAR or FORCE then it could be. On 9.2 I believe there is a bug that causes a reparse for every execution when SIMILAR is used so evey execution could be a different plan.
To check for skewed data just count the values in the column and then count by value. If the max(value_count) / count(*) is greater than a small percentage of the data then the column is skewed. The worse the skew the more likely that a plan chosen for a distinctive value is not a good plan for a non-distinctive value.
HTH -- Mark D Powell --