Updates to MDC tables

In an MDC table, updates of non-dimension values are done in place just as they are done with regular tables. If the update affects a variable length column and the record no longer fits on the page, another page with sufficient space is found.

The search for this new page begins within the same block. If there is no space in that block, the algorithm to insert a new record is used to find a page in the logical cell with enough space. There is no need to update the block indexes, unless no space is found in the cell and a new block needs to be added to the cell.

Updates of dimension values are treated as a delete of the current record followed by an insert of the changed record, because the record is changing the logical cell to which it belongs. If the deletion of the current record causes a block to be emptied, the block index needs to be updated. Similarly, if the insert of the new record requires it to be inserted into a new block, the block index needs to be updated.

Block indexes only need to be updated when inserting the first record into a block or when deleting the last record from a block. Index overhead associated with block indexes for maintenance and logging is therefore much less than the index overhead associated with regular indexes. For every block index that would have otherwise been a regular index, the maintenance and logging overhead is greatly reduced.

MDC tables are treated like any existing table; that is, triggers, referential integrity, views, and materialized query tables can all be defined upon them.

When you are reusing blocks that were recently made empty, a conditional Z lock on the block must be used to ensure that it is not currently being scanned by a UR scanner.