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.

The Bitmap Heap Scan reads pages from a bitmap created by the other operations, filtering out any rows that don't match the condition.

This has the advantage that the pages can be read in order, like in a sequential scan, avoiding random I/O and hopefully thus making the reads faster.

It also means that, like in an index scan, only some pages need to be read rather than all of them.

If the bitmap does not fit in memory, defined by work_mem, Postgres will make some blocks lossy.

Bitmap Heap Scan fields

• Parent Relationship

• Parallel Aware

• Relation Name

• Schema

• Alias

• Startup Cost

• Total Cost

• Plan Rows

• Plan Width

• Actual Startup Time

• Actual Total Time

• Actual Rows

• Actual Loops

• Output

• Recheck Cond

• Rows Removed by Index Recheck

• Filter

• Rows Removed by Filter

• Exact Heap Blocks

• Lossy Heap Blocks

• 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 Index Scan

• BitmapAnd

• BitmapOr

• Parallel Bitmap Heap Scan

Related official docs

• Combining Multiple Indexes

• Query Planning

• Row Estimation Examples

• Using EXPLAIN

Further reading

• Configuring work_mem – pgMustard

• What is a Bitmap Heap Scan? – Stack Overflow

• Multi-Column Indexes – pgMustard