Scan Types in PostgreSQL

Warda Bibi lays out four classes of scan in PostgreSQL:

To understand how PostgreSQL scans data, we first need to understand how PostgreSQL stores it.

• A table is stored as a collection of 8KB pages (by default) on disk.
• Each page has a header, an array of item pointers (also called line pointers), and the actual tuple data growing from the bottom up.
• Each tuple has its own header containing visibility info: xmin, xmax, cmin/cmax, and infomask bits.

There are different ways PostgreSQL can read data from disk. Depending on the query and available indexes, it can choose from several scan strategies:

1. Sequential Scan 
2. Index Scan
3. Index-Only Scan
4. Bitmap Index Scan

Read on for a description of those types, as well as when it makes sense for the database engine to select a particular scan type.