What is the difference between LATERAL and a subquery in PostgreSQL?
What is a LATERAL
join?
The feature was introduced with PostgreSQL 9.3.
Quoting the manual:
Subqueries appearing in
FROM
can be preceded by the key wordLATERAL
. This allows them to reference columns provided by precedingFROM
items. (WithoutLATERAL
, each subquery is evaluated independently and so cannot cross-reference any otherFROM
item.)Table functions appearing in
FROM
can also be preceded by the key wordLATERAL
, but for functions the key word is optional; the function's arguments can contain references to columns provided by precedingFROM
items in any case.
Basic code examples are given there.
More like a correlated subquery
A LATERAL
join is more like a correlated subquery, not a plain subquery, in that expressions to the right of a LATERAL
join are evaluated once for each row left of it - just like a correlated subquery - while a plain subquery (table expression) is evaluated once only. (The query planner has ways to optimize performance for either, though.)
Related answer with code examples for both side by side, solving the same problem:
- Optimize GROUP BY query to retrieve latest row per user
For returning more than one column, a LATERAL
join is typically simpler, cleaner and faster.
Also, remember that the equivalent of a correlated subquery is LEFT JOIN LATERAL ... ON true
:
- Call a set-returning function with an array argument multiple times
Things a subquery can't do
There are things that a LATERAL
join can do, but a (correlated) subquery cannot (easily). A correlated subquery can only return a single value, not multiple columns and not multiple rows - with the exception of bare function calls (which multiply result rows if they return multiple rows). But even certain set‑returning functions are only allowed in the FROM
clause. Like unnest()
with multiple parameters in Postgres 9.4 or later. The manual:
This is only allowed in the
FROM
clause;
So this works, but cannot (easily) be replaced with a subquery:
CREATE TABLE tbl (a1 int[], a2 int[]);
SELECT * FROM tbl, unnest(a1, a2) u(elem1, elem2); -- implicit LATERAL
The comma (,
) in the FROM
clause is short notation for CROSS JOIN
.
LATERAL
is assumed automatically for table functions.
About the special case of UNNEST( array_expression [, ... ] )
:
- How do you declare a set-returning-function to only be allowed in the FROM clause?
Set-returning functions in the SELECT
list
You can also use set-returning functions like unnest()
in the SELECT
list directly. This used to exhibit surprising behavior with more than one such function in the same SELECT
list up to Postgres 9.6. But it has finally been sanitized with Postgres 10 and is a valid alternative now (even if not standard SQL). See:
- What is the expected behaviour for multiple set-returning functions in SELECT clause?
Building on above example:
SELECT *, unnest(a1) AS elem1, unnest(a2) AS elem2
FROM tbl;
Comparison:
dbfiddle for pg 9.6 here
dbfiddle for pg 10 here
Clarify misinformation
The manual:
For the
INNER
andOUTER
join types, a join condition must be specified, namely exactly one ofNATURAL
,ON
join_condition, orUSING
(join_column [, ...]). See below for the meaning.
ForCROSS JOIN
, none of these clauses can appear.
So these two queries are valid (even if not particularly useful):
SELECT *
FROM tbl t
LEFT JOIN LATERAL (SELECT * FROM b WHERE b.t_id = t.t_id) t ON TRUE;
SELECT *
FROM tbl t, LATERAL (SELECT * FROM b WHERE b.t_id = t.t_id) t;
While this one is not:
SELECT *
FROM tbl t
LEFT JOIN LATERAL (SELECT * FROM b WHERE b.t_id = t.t_id) t;
That's why Andomar's code example is correct (the CROSS JOIN
does not require a join condition) and Attila's is was not.
The difference between a non-lateral
and a lateral
join lies in whether you can look to the left hand table's row. For example:
select *
from table1 t1
cross join lateral
(
select *
from t2
where t1.col1 = t2.col1 -- Only allowed because of lateral
) sub
This "outward looking" means that the subquery has to be evaluated more than once. After all, t1.col1
can assume many values.
By contrast, the subquery after a non-lateral
join can be evaluated once:
select *
from table1 t1
cross join
(
select *
from t2
where t2.col1 = 42 -- No reference to outer query
) sub
As is required without lateral
, the inner query does not depend in any way on the outer query. A lateral
query is an example of a correlated
query, because of its relation with rows outside the query itself.
First, Lateral and Cross Apply is same thing. Therefore you may also read about Cross Apply. Since it was implemented in SQL Server for ages, you will find more information about it then Lateral.
Second, according to my understanding, there is nothing you can not do using subquery instead of using lateral. But:
Consider following query.
Select A.*
, (Select B.Column1 from B where B.Fk1 = A.PK and Limit 1)
, (Select B.Column2 from B where B.Fk1 = A.PK and Limit 1)
FROM A
You can use lateral in this condition.
Select A.*
, x.Column1
, x.Column2
FROM A LEFT JOIN LATERAL (
Select B.Column1,B.Column2,B.Fk1 from B Limit 1
) x ON X.Fk1 = A.PK
In this query you can not use normal join, due to limit clause. Lateral or Cross Apply can be used when there is not simple join condition.
There are more usages for lateral or cross apply but this is most common one I found.