This is a feature that I have come across on certain versions of MSSQL and
basically involves a condition clause being placed on an index and records
are only indexed if they meet the condition, so
CREATE INDEX IDXtest ON tablex (value1, value2) WHERE value2 = 2
The value of such a feature includes
1) Reduced I/O maintaining the index file if only a small percentage of
the data conforms to the condition.
2) Reduced storage space requirement depending on the number of records
that conform to the condition.
3) A level of performance gain on data reads as a smaller index tree
has to be accessed.
The times I found this the most useful was when a record may have a number
of 'states' that it may be in. Certain states may be only very small sets
from the total data set, but a standard index must
record all the records in the data set, even if you never want to access
certain states - or those states were best handled by a table scan.
Thanks in advanced
Roger
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