PostgreSQL 10 High Performance - Third Edition

Materialize

Normally, nodes return their output row when their parent node asks for them. Sometimes when executing a sub-select, or inside part of a join, the planner may consider it more efficient to materialize that node instead. This produces the entire row set at once instead of having each upper-limit row grab them. This is rare enough that it's hard to even show a good example of it.

A much more common use of materialize involves merge joins, and in some cases nested loops, and those are specifically covered in a later section.