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While thinking about TYPO3 performance, I remembered one question asked about TYPO3 and phpMyAdmin.
phpMyAdmin complains that some TYPO3 tables use the same field in more than one index. Why does it complain?
Firsts, it must be stressed out that there is no error here. It is perfectly legal to use the same field in several indexes. If there are several complex queries, composite indexes (i.e. indexes that have several fields in them), can make these queries faster. It is useless to put fields in separate indexes: MySQL always uses only one index per query (with exception to UNION queries). So composite indexes are very useful. But composite indexes often include the same field (for example, crdate, tstamp or sorting fields to sort items). This perfectly fine.
The reason why phpMyAdmin complains is simple: it warns you about potential waste of index disk space. Imagine that you have a varchar(255) URL field in 10 indexes. This is up to 2550 bytes for each record. Or 25M for 10000 records only for this field.
But if you are sure you need the same field in many indexes - use it! It is not a error.
