SQL explain plan: what is Materialize?

Question

I asked PostgreSQL to explain my query. Part of the explanation was:

table_name --> Materialize

What does materialize do? I\'m joining t

Answer 1

In merge join and nested loop join, the database will "rescan" the inner loop. Basically like:

for each row in outer table:
    for each row in inner table:
        # do something

The planner will materializes the inner loop table which means load the whole table in an in-memory buffer to avoid the expensive disk IO cost.

A useful link.