|PostgreSQL 8.2.6 Documentation|
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In some situations it is worthwhile to rebuild indexes periodically with the REINDEX command.
In PostgreSQL releases before 7.4, periodic reindexing was frequently necessary to avoid "index bloat" , due to lack of internal space reclamation in B-tree indexes. Any situation in which the range of index keys changed over time — for example, an index on timestamps in a table where old entries are eventually deleted — would result in bloat, because index pages for no-longer-needed portions of the key range were not reclaimed for re-use. Over time, the index size could become indefinitely much larger than the amount of useful data in it.
In PostgreSQL 7.4 and later, index pages that have become completely empty are reclaimed for re-use. There is still a possibility for inefficient use of space: if all but a few index keys on a page have been deleted, the page remains allocated. So a usage pattern in which all but a few keys in each range are eventually deleted will see poor use of space. For such usage patterns, periodic reindexing is recommended.
The potential for bloat in non-B-tree indexes has not been well characterized. It is a good idea to keep an eye on the index's physical size when using any non-B-tree index type.
Also, for B-tree indexes a freshly-constructed index is somewhat faster to access than one that has been updated many times, because logically adjacent pages are usually also physically adjacent in a newly built index. (This consideration does not currently apply to non-B-tree indexes.) It might be worthwhile to reindex periodically just to improve access speed.