Encoded vector index
An encoded vector index is a permanent object that provides access to a table by assigning codes to distinct key values and then representing those values in a vector.
The size of the vector will match the number of rows in the underlying table. Each vector entry represents the table row number in the same position. The codes generated to represent the distinct key values can be 1, 2 or 4 bytes in length, depending upon the number of distinct values that need to be represented. Because of their compact size and relative simplicity, the EVI can be used to process large amounts of data very efficiently.
Even though an encoded vector index is used to represent the values stored in a table, the index itself cannot be used to directly gain access to the table. Instead, the encoded vector index can only be used to generate either a temporary row number list or a temporary row number bitmap. These temporary objects can then be used in conjunction with a Table Probe to specify the rows in the table that the query needs to process. The main difference with the Table Probe associated with an encoded vector index (versus a radix index) is that the paging associated with the table can be asynchronous. The I/O can now be scheduled more efficiently to take advantage of groups of selected rows. Large portions of the table can be skipped over where no rows are selected.
Visual explain icon:
- Encoded vector index symbol table scan
An encoded vector index symbol table scan operation is used to retrieve the entries from the symbol table portion of the index. - Encoded vector index probe
The encoded vector index (EVI) is quickly probed based upon the selection criteria that were rewritten into a series of ranges. It produces either a temporary row number list or bitmap.