On what circumstances will the query not use the index on a column? For example, I have an index on column A and if I use operator <> or IN(), will it use the index or full table scan?
In general, the query optimizer avoids using an index for scanning when the selection criteria returns a large number of rows (e.g., greater than 10 or 20% of the rows are selected). Read the Indexing Strategy white paper online for more details here.
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