http://stackoverflow.com/questions/18783/favourite-performance-tuning-tricks
Here is the handy-dandy list of things I always give to someone asking me about optimisation.
We mainly use Sybase, but most of the advice will apply across the board.
SQL Server, for example, comes with a host of performance monitoring / tuning bits, but if you don't have anything like that (and maybe even if you do) then I would consider the following...
99% of problems I have seen are caused by putting too many tables in a join. The fix for this is to do half the join (with some of the tables) and cache the results in a temporary table. Then do the rest of the query joining on that temporary table.
Query Optimisation Checklist
- Run UPDATE STATISTICS on the underlying tables
- Many systems run this as a scheduled weekly job
- Delete records from underlying tables (possibly archive the deleted records)
- Consider doing this automatically once a day or once a week.
- Rebuild Indexes
- Rebuild Tables (bcp data out/in)
- Dump / Reload the database (drastic, but might fix corruption)
- Build new, more appropriate index
- Run DBCC to see if there is possible corruption in the database
- Locks / Deadlocks
- Ensure no other processes running in database
- Especially DBCC
- Are you using row or page level locking?
- Lock the tables exclusively before starting the query
- Check that all processes are accessing tables in the same order
- Are indices being used appropriately?
- Joins will only use index if both expressions are exactly the same data type
- Index will only be used if the first field(s) on the index are matched in the query
- Are clustered indices used where appropriate?
- range data
- WHERE field between value1 and value2
- Small Joins are Nice Joins
- By default the optimiser will only consider the tables 4 at a time.
- This means that in joins with more than 4 tables, it has a good chance of choosing a non-optimal query plan
- Break up the Join
- Can you break up the join?
- Pre-select foreign keys into a temporary table
- Do half the join and put results in a temporary table
- Are you using the right kind of temporary table?
- #temp tables may perform much better than @table variables with large volumes (thousands of rows).
- Maintain Summary Tables
- Build with triggers on the underlying tables
- Build daily / hourly / etc.
- Build ad-hoc
- Build incrementally or teardown / rebuild
- See what the query plan is with SET SHOWPLAN ON
- See what’s actually happenning with SET STATS IO ON
- Force an index using the pragma: (index: myindex)
- Force the table order using SET FORCEPLAN ON
- Parameter Sniffing:
- Break Stored Procedure into 2
- call proc2 from proc1
- allows optimiser to choose index in proc2 if @parameter has been changed by proc1
- Can you improve your hardware?
- What time are you running? Is there a quieter time?
- Is Replication Server (or other non-stop process) running? Can you suspend it? Run it eg. hourly?