What is LATERAL, what is it for, and how can one use it?

Lately in couple of places I recommended people that they can solve their problem with queries using LATERAL. In some cases recipient of such suggestion indicated that they had no idea what LATERAL is. Which made me think that it might be good idea to write more about them (lateral queries)…

Also – I know that some of the examples I shown in here can be done differently, I just wanted to show how one can use LATERAL, and am terrible with coming up with better usecases.

Continue reading What is LATERAL, what is it for, and how can one use it?

Explaining the unexplainable – part 3

In previous post in the series I wrote about how to interpret single line in explain analyze output, it's structure, and later on described all basic data-getting operations (nodes in explain tree).

Today, we'll move towards more complicated operations.

Continue reading Explaining the unexplainable – part 3

Waiting for 9.3 – Implement SQL-standard LATERAL subqueries.

On 7th of August, Tom Lane committed patch:

Implement SQL-standard LATERAL subqueries.
This patch implements the standard syntax of LATERAL attached to a
sub-SELECT in FROM, and also allows LATERAL attached to a function in FROM,
since set-returning function calls are expected to be one of the principal
The main change here is a rewrite of the mechanism for keeping track of
which relations are visible for column references while the FROM clause is
being scanned.  The parser "namespace" lists are no longer lists of bare
RTEs, but are lists of ParseNamespaceItem structs, which carry an RTE
pointer as well as some visibility-controlling flags.  Aside from
supporting LATERAL correctly, this lets us get rid of the ancient hacks
that required rechecking subqueries and JOIN/ON and function-in-FROM
expressions for invalid references after they were initially parsed.
Invalid column references are now always correctly detected on sight.
In passing, remove assorted parser error checks that are now dead code by
virtue of our having gotten rid of add_missing_from, as well as some
comments that are obsolete for the same reason.  (It was mainly
add_missing_from that caused so much fudging here in the first place.)
The planner support for this feature is very minimal, and will be improved
in future patches.  It works well enough for testing purposes, though.
catversion bump forced due to new field in RangeTblEntry.

Continue reading Waiting for 9.3 – Implement SQL-standard LATERAL subqueries.