Character Indexes

The maximum length for an index key equals one quarter of the page size, i.e. from 1,024 — for page size 4,096 — to 8,192 bytes — for page size 32,768.The maximum length of an indexed string is 9 bytes less than that quarter-page limit.

Calculating Maximum Length of an Indexed String Field

The following formula calculates the maximum length of an indexed string (in characters):

max_char_length = FLOOR((page_size / 4 - 9) / N)

where N is the number of bytes per character in the character set.

The table below shows the maximum length of an indexed string (in characters), according to page size and character set, calculated using this formula.

Table 1. Maximum Index Lengths by Page Size and Character Size
Page Size	Bytes per character
Page Size	1	2	3	4	6
4,096	1,015	507	338	253	169
8,192	2,039	1,019	679	509	339
16,384	4,087	2,043	1,362	1,021	681
32,768	8,183	4,091	2,727	2,045	1,363

Note	With case-insensitive collations (“_CI”), one character in the index key will occupy not 4, but 6 (six) bytes, so the maximum key length for a page of — for example — 4,096 bytes, will be 169 characters.

Character Indexes

Character Indexes

Character Types in Detail