What is bitmap index scan in PostgreSQL?

A Bitmap Heap Scan is the most common parent node of a Bitmap Index Scan, but a plan may also combine several different Bitmap Index Scans with BitmapAnd or BitmapOr nodes before actually fetching the underlying data. This allows Postgres to use two different indexes at once to execute a query.

Does PostgreSQL support bitmap indexes?

PostgreSQL is not provide persistent bitmap index. But it can be used in database to combine multiple indexes. PostgreSQL scans each needed index and prepares a bitmap in memory giving the locations of table rows that are reported as matching that index’s conditions.

What is bitmap index in SQL?

A bitmap index is a special kind of database index that uses bitmaps. Bitmap indexes have traditionally been considered to work well for low-cardinality columns, which have a modest number of distinct values, either absolutely, or relative to the number of records that contain the data.

What is an index scan?

An index scan occurs when the database manager accesses an index to narrow the set of qualifying rows (by scanning the rows in a specified range of the index) before accessing the base table; to order the output; or to retrieve the requested column data directly ( index-only access ).

Is seq scan bad?

A sequential scan is therefore not always bad – there are use cases, where a sequential scan is actually perfect. Still: Keep in mind that scanning large tables sequentially too often will take its toll at some point.

What is index only scan?

The Index-Only Scan is an index scan without subsequent table access—hence, accessing the index only. All recent versions of Oracle, SQL Server and MySQL support index-only scans. The PostgreSQL database supports index-only scans since release 9.2.

What is bitmap index scan?

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.

Why does PostgreSQL perform sequential scan on indexed column?

A lot of the times statistics are not updated on a table and it may not be possible to do so due to constraints. In this case, the optimizer will not know how many rows it should take in year > 2019. Thus it selects a sequential scan in lieu of full knowledge.

When should we use bitmap index?

In reality, a bitmap index is always advisable for systems in which data is not frequently updated by many concurrent systems. In fact, as I’ll demonstrate here, a bitmap index on a culumn with 100-percent unique values (a culumn candidate for primary key) is as efficient as a B-tree index.