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    <p>Actually there can be a decrease in performance when you drop the index on the single column.</p> <p>Suppose you do a query with <code>WHERE [Column1] = 123</code>. This could be solved perfectly with the original index. The new index can also be used for this but will -depending on the implementation- need to read the values for [Column2] in the index as well even though they are not used.</p> <p>So yes: there can in theory be a downside to dropping the index: increased reads.</p> <p>[Update 9 sept.]<br> Recently I encountered another situation where a single index can be <strong>much better</strong> than a combined index.<br> Consider a huge table 'bugs' with column [status], [createdate] and some other fields. Most bugs will be closed so assume status is '0' (open) for 100 records and '1' (closed) for 99000 records.</p> <p><code>SELECT * FROM bugs WHERE status = '0'</code> will benefit enormously from an index on <code>status</code> whereas with <code>SELECT * FROM bugs WHERE status = '1'</code> an index on <code>status</code> will not help.</p> <p>Oracle will know the difference because it builds statistics on the index.</p> <p>However, with an combined index on <code>status, createdate</code> each index entry is almost unique and Oracle will decide not to use the index for the query <code>SELECT * FROM bugs WHERE status = '0'</code> because it guesses (wrongly) that the index would not help.</p> <p>So in this situation a single index should not be dropped just because it is a prefix to a combined index.</p> <p>Note: In theory Oracle could build even smarter statistics on the index but it does not seem to do that.</p>
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    1. POIs it necessary to consider dropping the existing index because it is a prefix of the recommended index
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    1. USJeff
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    1. COIs there a link to this in any Oracle documentation? What sort of performance drop would be expected?
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    2. CODon't know of any documentation, just my interpretation of how I understand how indexes work. Having two columns in the index makes the total index twice as large. When the index is very selective (or unique) Oracle will usually only need to read a few 'blocks' to get all the information it needs. When the index is twice as large there is a chance it needs to read more blocks. However, I think the effect is negligible in all practical cases.
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    3. CO+1. I thought this was obvious, I'm surprised you haven't gotten more votes. If your index is bigger, and contains information that not every query will need, then certainly there will be a performance penalty in some cases.
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      1. USJon Heller
 

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