{snip}
Why would the index I have created not being used?
>>
The index you have created will not be used in several circumstances. For instance:
1. It is faster to do a table scan than to use the index, despite up to date statistics
2. The distribution of the data has changed since the last time you analyzed the database
Consider a truly horrible case, an index on a single character. As it turns out, this field contains exactly two values: ‘M’ or ‘F’ for male or female. About 50% of the data is ‘M’ and about 50% is ‘F’. If we were to use this index to scan the data, we will be loading the index pages, and then popping all over the data pages following the index. It will truly be an awful sight. We would spend far more effort than simply doing a table scan. Fortunately, we have statistics which have come to our rescue. They will tell the optimizer to simply ignore the horribly defined index file and never use it in any circumstance.
Consider an even more horrible case, the same index, but we have not updated statistics in months and we have automatic stats and vacuum disabled. The only time statistics was run, there was a single ‘F’ in the index and 44 ‘M’ values. A query comes along looking for “sex = ‘F’” and the optimizer decides to use the index. We can’t blame the poor optimizer. It’s not his fault that statistical collection was disabled. So he merrily informs the query planner to follow the index to collect the data, and the query takes eons to complete.
In short, using the index is not always a good idea. It’s a good idea to use an index when it is faster than not using an index. If you were to post the explain analyze output, experts here could tell you exactly why the decisions were made to use an index or not to use an index. And if an index should have been used, they can tell you what to do so that the index will be used next time.
<<
