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Maximum index key sizeEach database imposes a maximum size for index keys, also known as composite
indexes. For Microsoft SQL Server 2000The maximum size of an index is 900 bytes. The maximum number of fields in an index key is 16. To determine the size of a composite index key, SQL Server totals all
the fixed columns plus the minimums of all the variable columns to determine
if the composite index is within the allowable maximum size of an index.
However, if there is already data in the table, and if the existing data
in a column allows the total bytes of the index to be greater than the
maximum size, then the index cannot be created. Also, if after the index
is created data is inserted (or updated) that allows the maximum value
of the index to exceed the maximum value size, then an error occurs at
that time. For Oracle 9iThe maximum total size of an index key is 70% of the database block size minus some overhead. To determine the database data block size in SQLPlus:
To determine the size of an index key, the Oracle server totals the maximum defined sizes of all the variable length fields plus the sizes of all the fixed length fields. The maximum number of fields in an index key is 32.
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