Favourite performance tuning tricks

Question

When you have a query or stored procedure that needs performance tuning, what are some of the first things you try?

Answer 1

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?