Re: Question about semantics of $ variables in json explain plans in 13

Jerry Brenner <jbrenner(at)guidewire(dot)com> writes:
> Is there any documentation on the semantics of $ variables in json explain
> plans for both InitPlans and SubPlans in 13?

I don't think there's anything much in the user-facing docs, which is
somewhat unfortunate because it's confusing: the notation is overloaded.
$N could be a parameter supplied from outside the query (as in your $1,
$2 and $3 in the source text), but it could also be a parameter supplied
from an outer query level to a subplan, or it could be the result value
of an InitPlan. The numbering of outside-the-query parameters is
disjoint from that of the other kind.

> - It looks like $0 represents the value from the outer query block when
> the correlated subquery is evaluated
> - It looks like $1 represents the result of the subquery evaluation

Yeah, I think you're right here. $0 evidently corresponds to
qRoot.contactID from the outer plan, and the plan label itself
shows that $1 carries the sub-select's value back out. This $1
is unrelated to the $1 you wrote in the query text. (It looks
like this is a custom plan in which "67" was explicitly substituted
for your $3. Presumably $1 and $2 were replaced as well; we don't
do half-custom plans.)

regards, tom lane