pgsql: Add an alternative_plan_name field to PlannerInfo.

Add an alternative_plan_name field to PlannerInfo.

Typically, we have only one PlannerInfo for any given subquery, but
when we are considering a MinMaxAggPath or a hashed subplan, we end
up creating a second PlannerInfo for the same portion of the query,
with a clone of the original range table. In fact, in the MinMaxAggPath
case, we might end up creating several clones, one per aggregate.

At present, there's no easy way for a plugin, such as pg_plan_advice,
to understand the relationships between the original range table and
the copies of it that are created in these cases.  To fix, add an
alternative_plan_name field to PlannerInfo. For a hashed subplan, this
is the plan name for the non-hashed alternative; for minmax aggregates,
this is the plan_name from the parent PlannerInfo; otherwise, it's the
same as plan_name.

Discussion: http://postgr.es/m/CA+TgmoYuWmN-00Ec5pY7zAcpSFQUQLbgAdVWGR9kOR-HM-fHrA@mail.gmail.com
Reviewed-by: Lukas Fittl <lukas@fittl.com>

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/26255a320733de2d91a30bd6ae529dd01e7f3409

Modified Files
--------------
src/backend/optimizer/path/allpaths.c     |  2 +-
src/backend/optimizer/plan/planagg.c      |  1 +
src/backend/optimizer/plan/planner.c      | 15 +++++++++++----
src/backend/optimizer/plan/subselect.c    |  6 +++---
src/backend/optimizer/prep/prepjointree.c |  1 +
src/backend/optimizer/prep/prepunion.c    |  2 +-
src/include/nodes/pathnodes.h             | 12 ++++++++++++
src/include/optimizer/planner.h           |  1 +
8 files changed, 31 insertions(+), 9 deletions(-)