| CPI4338 | |
|---|---|
| Message Text: | &1 Access path(s) used for bitmap processing of file &2. |
| Cause Text: | Bitmap processing was used to access records from member &4 of file &2 in library &3. Bitmap processing is a method of allowing one or more access path(s) to be used to access the selected records from a file. Using bitmap processing, record selection is applied against each access path, similar to key row positioning, to create a bitmap. The bitmap has marked in it only the records of the file that are to be selected. If more than one access path is used, the resulting bitmaps are merged together using boolean logic. The resulting bitmap is then used to reduce access to just those records actually selected from the file. Bitmap processing is used in conjunction with the two primary access methods: arrival sequence (CPI4327 or CPI4329) or keyed access (CPI4326 or CPI4328). The message that describes the primary access method immediately precedes this message. When the bitmap is used with the keyed access method then it is used to further reduce the number of records selected by the primary access path before retrieving the selected records from the file. When the bitmap is used with arrival sequence then it allows the sequential scan of the file to skip records which are not selected by the bitmap. This is called skip sequential processing. The list below shows the names of the access paths used in the bitmap processing: If file &2 in library &3 is a logical file then member &7 of physical file &5 in library &6 is the actual file being accessed. |
| Recovery Text: | Refer to Data access on DB2 UDB for iSeries: data access paths and methods for more information about bitmap processing. |
The optimizer chooses to use one or more indexes, in conjunction with the query selection (WHERE clause), to build a bitmap. This resulting bitmap indicates which rows will actually be selected.
Conceptually, the bitmap contains one bit per row in the underlying table. Corresponding bits for selected rows are set to '1'. All other bits are set to '0'.
Once the bitmap is built, it is used, as appropriate, to avoid mapping in rows from the table not selected by the query. The use of the bitmap depends on whether the bitmap is used in combination with the arrival sequence or with a primary index.
When bitmap processing is used with arrival sequence, either message CPI4327 or CPI4329 will precede this message. In this case, the bitmap will help to selectively map only those rows from the table that the query selected.
When bitmap processing is used with a primary index, either message CPI4326 or CPI4328 will precede this message. Rows selected by the primary index will be checked against the bitmap before mapping the row from the table.