Get the (zero based) id of the column to be qualified.
This id is the column number of the column in the table, no matter
whether a partial column set is being retrieved by the actual fetch.
Note that the column being specified in the qualifier must appear in
the column list being fetched.
Clear the DataValueDescriptor cache, if one exists.
(The DataValueDescriptor can be 1 of 3 types:
o VARIANT - cannot be cached as its value can
vary within a scan
o SCAN_INVARIANT - can be cached within a scan as its
value will not change within a scan
o QUERY_INVARIANT- can be cached across the life of the query
as its value will never change
o CONSTANT - immutable
This method reinitializes all the state of
the Qualifier. It is used to distinguish between
resetting something that is query invariant
and something that is constant over every
execution of a query. Basically, clearOrderableCache()
will only clear out its cache if it is a VARIANT
or SCAN_INVARIANT value. However, each time a
query is executed, the QUERY_INVARIANT qualifiers need
to be reset.