GROUP_BITMAP_MERGE

Introduction

The GROUP_BITMAP_MERGE function is used to merge multiple intermediate Bitmap objects and returns the cardinality of the merged result.

Syntax

group_bitmap_merge(bitmap_state)

Parameters

• bitmap_state: An intermediate BITMAP object, representing the Bitmap state to be merged.

Return Value

The function returns a result of type INT, representing the cardinality of the merged Bitmap.

Example

Assume there is a table bitmap_column_expr_test2 with a column z that stores multiple intermediate Bitmap data. The query returns the cardinality of the merged Bitmap:

SELECT group_bitmap_merge(bitmapBuild(v)) AS res FROM VALUES (array(1,2,3,4,5,6,7,8,9,10)), (array(6,7,8,9,10,11,12,13,14,15)), (array(2,4,6,8,10,12)) AS t(v); +-----+ | res | +-----+ | 15 | +-----+

Example 2: Two-stage aggregation with group_bitmap_state

First use group_bitmap_state to generate intermediate state by group, then use group_bitmap_merge to merge all groups and obtain the global cardinality:

SELECT group_bitmap_merge(state) AS total FROM ( SELECT c, group_bitmap_state(v) AS state FROM VALUES ('a', 1), ('a', 2), ('b', 2), ('b', 3) AS t(c, v) GROUP BY c ); +-------+ | total | +-------+ | 3 | +-------+

Notes

1. Input Type: Ensure the input bitmap_state is of type BITMAP.
2. Performance Optimization: This function is highly efficient for large-scale datasets. However, performance impact should still be considered if the data volume is extremely large.