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Limits
Firebird allows up to 256 user-created indexes per table in version 1.5 and higher, or
64 in lower releases. However, these are theoretical limits that are governed by both
page size and the actual on-disk size of the index description data on the index root
page. You could not store 256 indexes in a database with a page size smaller than 16KB.
On the index root page, each index needs 31 bytes for its identifier, space for descriptions
of each segment (column) involved in the index, and some bytes to store a pointer
to the first page of the index. Even a 16KB page may not be able to accommodate
256 indexes if there are more than a few compound indexes in the database.
To create indexes, a user must be authorized to connect to the database.
An index cannot exceed 253 bytes in total width. In reality, the number of bytes
may be significantly smaller. Factors that can reduce the number of actual “slots” available
to store characters include
• International character sets that use multiple bytes per character
• International character sets with complex uppercase/lowercase pairings and/or
dictionary sorting rules
• Use of non-binary collations
• Multiple segments (composite indexes) that require the addition of padding
bytes to retain the geometry of the index
In other words, using any character set except NONE will influence your decisions
about index design—particularly whether to use composite indexes. That’s the bad
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news. The good news is that Firebird makes good use of single-column indexes in
multi-column searches and sorts, reducing the need for many multi-column indexes
that you might have experienced with another DBMS.
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