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Available in: DSQL, ESQL

Description: Creates an index on a table for faster searching, sorting and/or grouping.


   ON tablename
   { (<col> [, <col> ...]) | COMPUTED BY (expression) }

<col>  ::=  a column not of type ARRAY, BLOB or COMPUTED BY

UNIQUE indices now allow NULLs

Changed in: 1.5

Description: In compliance with the SQL-99 standard, NULLs – even multiple – are now allowed in columns that have a UNIQUE index defined on them. For a full discussion, see CREATE TABLE :: UNIQUE constraints now allow NULLs. As far as NULLs are concerned, the rules for unique indices are exactly the same as those for unique keys.

Indexing on expressions

Added in: 2.0

Description: Instead of one or more columns, you can now also specify a single COMPUTED BY expression in an index definition. Expression indices will be used in appropriate queries, provided that the expression in the WHERE, ORDER BY or GROUP BY clause exactly matches the expression in the index definition. Multi-segment expression indices are not supported, but the expression itself may involve multiple columns.


create index ix_upname on persons computed by (upper(name));

-- the following queries will use ix_upname:
select * from persons order by upper(name);
select * from persons where upper(name) starting with 'VAN';
delete from persons where upper(name) = 'BROWN';
delete from persons where upper(name) = 'BROWN' and age > 65;
create descending index ix_events_yt
  on MyEvents
  computed by (extract(year from StartDate) || Town);

-- the following query will use ix_events_yt:
select * from MyEvents
  order by extract(year from StartDate) || Town desc;

Maximum index key length increased

Changed in: 2.0

Description: The maximum length of index keys, which used to be fixed at 252 bytes, is now equal to 1/4 of the page size, i.e. varying from 256 to 4096. The maximum indexable string length in bytes is 9 less than the key length. The table below shows the indexable string lengths in characters for the various page sizes and character sets.

Table 6.2. Maximum indexable (VAR)CHAR length

Page size Maximum indexable string length per charset type
1 byte/char 2 bytes/char 3 bytes/char 4 bytes/char
1024 247 123 82 61
2048 503 251 167 125
4096 1015 507 338 253
8192 2039 1019 679 509
16384 4087 2043 1362 1021

Maximum number of indices per table increased

Changed in: 1.0.3, 1.5, 2.0

Description: The maximum number of 65 indices per table has been removed in Firebird 1.0.3, reintroduced at the higher level of 257 in Firebird 1.5, and removed once again in Firebird 2.0.

Although there is no longer a hard ceiling, the number of indices creatable in practice is still limited by the database page size and the number of columns per index, as shown in the table below.

Table 6.3. Max. indices per table, Firebird 2.0

Page size Number of indices depending on column count
1 col 2 cols 3 cols
1024 50 35 27
2048 101 72 56
4096 203 145 113
8192 408 291 227
16384 818 584 454

Please be aware that under normal circumstances, even 50 indices is way too many and will drastically reduce mutation speeds. The maximum was removed to accommodate data-warehousing applications and the like, which perform lots of bulk operations with the indices temporarily inactivated.

For a full table also including Firebird versions 1.0–1.5, see the Notes at the end of the book.

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