Can Multiple Indexes Work Together?

Question

Suppose I have a database table with two fields, \"foo\" and \"bar\". Neither of them are unique, but each of them are indexed. However, rather than being indexed together, th

Answer 1

The best approach would be to add foo to bar's index, or add bar to foo's index (or both). If foo's index also contains an index on bar, that additional indexing level will not affect the utility of the foo index in any current uses of that index, nor will it appreciably affect the performance of maintaining that index, but it will give the database additional information to work with in optimizing queries such as in the example.

Answer 2

You can provide hints as to which index to use. I'm not familiar with Oracle, but in Mysql you can use USE|IGNORE|FORCE_INDEX (see here for more details). For best performance though you should use a combined index.

Answer 3

So is Oracle smart enough to search efficiently here?

The simple answer is "probably". There are lots'o' very bright people at each of the database vendors working on optimizing the query optimizer, so it's probably doing things that you haven't even thought of. And if you update the statistics, it'll probably do even more.

Answer 4

Eli,

In a comment you wrote:

Unfortunately, I have a table with lots of columns each with their own index. Users can query any combination of fields, so I can't efficiently create indexes on each field combination. But if I did only have two fields needing indexes, I'd completely agree with your suggestion to use two indexes. – Eli Courtwright (Sep 29 at 15:51)

This is actually rather crucial information. Sometimes programmers outsmart themselves when asking questions. They try to distill the question down to the seminal points but quite often over simplify and miss getting the best answer.

This scenario is precisely why bitmap indexes were invented -- to handle the times when unknown groups of columns would be used in a where clause.

Just in case someone says that BMIs are for low cardinality columns only and may not apply to your case. Low is probably not as small as you think. The only real issue is concurrency of DML to the table. Must be single threaded or rare for this to work.

Answer 5

It's better than that.

Index Seeks are always quicker than full table scans. So behind the scenes Oracle (and SQL server for that matter) will first locate the range of rows on both indices. It will then look at which range is shorter (seeing that it's an inner join), and it will iterate the shorter range to find the matches with the larger of the two.

Answer 6

First off, I'll assume that you are talking about nice, normal, standard b*-tree indexes. The answer for bitmap indexes is radically different. And there are lots of options for various types of indexes in Oracle that may or may not change the answer.

At a minimum, if the optimizer is able to determine the selectivity of a particular condition, it will use the more selective index (i.e. the index on bar). But if you have skewed data (there are N values in the column bar but the selectivity of any particular value is substantially more or less than 1/N of the data), you would need to have a histogram on the column in order to tell the optimizer which values are more or less likely. And if you are using bind variables (as all good OLTP developers should), depending on the Oracle version, you may have issues with bind variable peeking.

Potentially, Oracle could even do an on the fly conversion of the two b*-tree indexes to bitmaps and combine the bitmaps in order to use both indexes to find the rows it needs to retrieve. But this is a rather unusual query plan, particularly if there are only two columns where one column is highly selective.