Lateral subqueries in Postgres
Postgres has feature to use value from preceding table using keyword lateral. This is used for cross-referencing. means Subqueries appearing in FROM can be preceded by the key word LATERAL. This allows them to reference columns provided by preceding FROM items. (Without LATERAL, each subquery is evaluated independently and so cannot cross-reference any other FROM item.)
Lateral evaluation proceeds as follows: for each row of the FROM item providing the cross-referenced column(s), or set of rows of multiple FROM items providing the columns, the LATERAL item is evaluated using that row or row set’s values of the columns. The resulting row(s) are joined as usual with the rows they were computed from. This is repeated for each row or set of rows from the column source table(s).
For more information refer to this link
A trivial example of lateral
SELECT * FROM foo, LATERAL (SELECT * FROM bar WHERE bar.id = foo.bar_id) ss;
In the above SQL query , the table foo generates a row with id (foo.bar_id) now this id is passed to sub query where it generates the result.
It is often particularly handy to LEFT JOIN to a LATERAL subquery, so that source rows will appear in the result even if the LATERAL subquery produces no rows for them. For example, if get_product_names() returns the names of products made by a manufacturer, but some manufacturers in our table currently produce no products, we could find out which ones those are like this:
SELECT m.name FROM manufacturers m LEFT JOIN LATERAL get_product_names(m.id) pname ON true WHERE pname IS NULL;