Bitmap scans are a multi-step process that consist of a Bitmap Heap Scan, one or more Bitmap Index Scans and optionally BitmapOr and BitmapAnd operations.

A Bitmap Index Scan is the first step, using an index to create a bitmap of rows that may* fulfil (at least part of) the condition.

If the bitmap does not fit in memory, defined by work_mem, Postgres will make some blocks lossy, but this is shown on the Bitmap Heap Scan.

*Some index types, like BRIN, don't guarantee it.

Bitmap Index Scan fields

• Parent Relationship

• Parallel Aware

• Index Name

• Startup Cost

• Total Cost

• Plan Rows

• Plan Width

• Actual Startup Time

• Actual Total Time

• Actual Rows

• Actual Loops

• Index Cond

• Shared Hit Blocks

• Shared Read Blocks

• Shared Dirtied Blocks

• Shared Written Blocks

• Local Hit Blocks

• Local Read Blocks

• Local Dirtied Blocks

• Local Written Blocks

• Temp Read Blocks

• Temp Written Blocks

• Shared I/O Read Time

• Shared I/O Write Time

• Local I/O Read Time

• Local I/O Write Time

• Temp I/O Read Time

• Temp I/O Write Time



Related operations

• Bitmap Heap Scan

• BitmapAnd

• BitmapOr

Related official docs

• Combining Multiple Indexes

• Query Planning

• Row Estimation Examples

• Using EXPLAIN

Further reading

• What is a Bitmap Heap Scan? – Stack Overflow

• Multi-Column Indexes – pgMustard

• Why isn’t Postgres using my index? – pgMustard