Changing plans (hint hint)
 
 
- Most database systems provide a way to influence the execution plan
- Oracle supports a very large and complex range of hints
- 
- Hints must be contained within special comments /*+ … */
- 
 SELECT /*+ INDEX(table1 index1) */ foo, bar
 FROM table1 WHERE key1=1 AND key2=2 AND key3=3;
 
 
- MySQL has a very limited set of hints
- 
- Hints can optionally be placed inside comments /*! … */
- 
 SELECT foo, bar FROM table1 /*! USE INDEX (key1,key2) */
 WHERE key1=1 AND key2=2 AND key3=3;
- 
 
- Use sparingly! Generally as a last resort.
- 
- A hint may help now but later schema (or data) changes may make it worse.
- Usually best to let the optimizer do its job