A join is "lateral" when its right operand depends on the left operand.

Lateral joins are one of the ways a subquery or relational expression can be correlated.

Lateral joins occur:

• in the regular join syntax with the keyword "LATERAL", e.g. SELECT * FROM a, LATERAL b
• implicitly when using a SRF in the select render position and one of the SRF argument uses a column name from one of the existing FROM tables, e.g. SELECT generate_series(v) FROM kv
• also needed for common uses of jsonb_object_keys(), see sql: cannot project json_object_keys() on a json column #26110

Note: LATERAL in the join syntax changes the name resolution rules!

For example, the following two queries have very different behavior:

SELECT
   (SELECT a FROM db1.ab, LATERAL (SELECT * FROM kv WHERE v = b))
          -- v = b refers to "b" in db1.ab
FROM db2.ab

vs.

SELECT
   (SELECT a FROM db1.ab, (SELECT * FROM kv WHERE v = b))
          -- v = b refers to "b" in db2.ab!
FROM db2.ab