Star schema in data Warehouse

SQL Server query plan without bitmap filters.The bitmap filter compares favorably to the bitmap index. A bitmap index is an alternate form for representing row ID (RID) lists in a value-list index using one or more bit vectors indicating which row in a table contains a certain column value. Both can be very effective in removing unnecessary rows from result processing; however, there are important differences between a bitmap filter and a bitmap index. First, bitmap filters are in-memory structures, thus eliminating any index maintenance overhead due to data manipulation language (DML) operations made to the underlying table. In addition, bitmap filters are very small and, unlike existing on-disk indexes that typically depend on the size of the table on which they are built, bitmap filters can be created dynamically with minimal impact on query processing time.

Comparing Bitmap Filtering with Optimized Bitmap Filtering

Bitmap filtering and optimized bitmap filtering are implemented in the query plan by using the bitmap showplan operator. Bitmap filtering is applied only in parallel query plans in which hash or merge joins are used. Optimized bitmap filtering is applicable only to parallel query plans in which hash joins are used. In both cases, the bitmap filter is created on the build input (the dimension table) side of a hash join; however, the actual filtering is typically done within the Parallelism operator, which is on the probe input (the fact table) side of the hash join. When the join is based on an integer column, the filter can be applied directly to the initial table or index scan operation rather than the Parallelism operator. This technique is called in-row optimization.

When bitmap filtering is introduced in the query plan after optimization, query compilation time is reduced; however, the query plans that the optimizer can consider are limited, and cardinality and cost estimates are not taken into account.

Optimized bitmap filters have the following advantages:

  • Filtering from several dimension tables is supported.
  • Multiple filters can be applied to a single operator.
  • Optimized bitmap filters can be applied to more operator types. These include exchange operators such as the Distribute Streams and Repartition Streams operators, table or index scan operators, and filter operators.
  • Filtering is applicable to SELECT statements and the read-only operators used in INSERT, UPDATE, DELETE, and MERGE statements.
  • Filtering is applicable to the creation of indexed views in the operators used to populate the index.
  • The optimizer uses cardinality and cost estimates to determine if optimized bitmap filtering is appropriate.
  • The optimizer can consider more plans.

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