It not make sense, a literal is not a valid column?
SELECT x, y FROM (select 1 as x) t, LATERAL CAST(2 AS FLOAT) AS y; -- fine
SELECT x, y FROM (select 1 as x) t, LATERAL 2.0 AS y; -- SYNNTAX ERROR!
Same if you use CASE
clause or x+1
expression or (x+1)
… seems ERROR for any non-function.
The Pg Guide, about LATERAL expression
(not LATERAL subquery), say
LATERAL is primarily useful when the cross-referenced column is necessary for computing the row(s) to be joined (…)
NOTES
The question is about LATERAL 1_column_expression
not LATERAL multicolumn_subquery
. Example:
SELECT x, y, exp, z
FROM (select 3) t(x), -- subquery
LATERAL round(x*0.2+1.2) as exp, -- expression!
LATERAL (SELECT exp+2.0 AS y, x||'foo' as z) t2 --subquery
;
… After @klin comment showing that the Guide in another point say "only functions", the question Why? must be expressed in a more specific way, changing a litle bit the scope of the question:
Not make sense "only funcions", the syntax (x)
or (x+1)
, encapsulatening expression in parentesis, is fine, is not?
Why only functions?
PS: perhaps there is a future plan, or perhaps a real problem on parsing generic expressions… As users we must show to PostgreSQL developers what make sense and we need.
3
Answers
Answering "Why only functions?" by intuition.
Or "Why does the PostgreSQL spec use only functions?". Of course, it's not a question about the parser, because it complies with the specification.
The
SELECT
syntax Guide show the only occasions when we can useLATERAL
:So, no conflict on
The guess of @Bergi is that a literal expression like
LATERAL 2.0 AS y
could be interpreted asLATERAL "2"."0"
, the "table 0" and "schema 2"... But, as we saw above, not make sense to expect a table name after clauseLATERAL
, so, in fact, no ambiguity.Conclusion: it looks like the specification of
LATERAL
can grow and allow the use of expressions.This is the great advantage of being able to discuss and participate in an open community software!
Why
LATERAL single_expression AS alias
? Rationale:to be orthogonal: any new user of PostgreSQL, that see that is valid
SELECT a, x, x+b AS y FROM t, LATERAL f(a) AS x
, will naturally try also expressions instead functions. It is expected in a "orthogonal system" and is intuitive for any programmer.to reuse expressions: we use "chain of dependent expressions" in any language, things like
a=b+c; x=a+y; z=a/2; ...
. It is ugly to do "SELECT(SELECT(SELECT))" in SQL, only for reuse expressions. The "chains of LATERALs" is more elegant and human-readable.And perhaps is better also for query optimization.
It’ll all work fine if you wrap it in its own subquery
A literal is a valid value for a column, but as the docs you quoted say,
LATERAL
syntax is usedA relation, such as a
FROM
orJOIN
orLATERAL
subquery clause, always computes tuples of (a single or multiple) columns. The alias you’re assigning is not for an individual row, but for the whole tuple.