Re: no partition pruning when partitioning using array type

Another thing I realized when testing this is that partitioning over a
domain doesn't work very nicely (tested in 10 and master):

create domain overint as int;
create table pt (a overint) partition by range (a);
create table pt1 partition of pt for values from (0) to (100);

results in:

ERROR:  specified value cannot be cast to type overint for column "a"
LÍNEA 1: create table pt1 partition of pt for values from (0) to (100...
                                                           ^
DETALLE:  The cast requires a non-immutable conversion.
SUGERENCIA:  Try putting the literal value in single quotes.

I tried to do what the HINT says, but it fails in the same way.  I also
tried to add casts, but those are rejected as syntax errors.

Tracing it down, turns out that transformPartitionBoundValue gets from
coerce_to_target_type a CoerceToDomain node.  It then tries to apply
expression_planner() to simplify the expression, but that one doesn't
want anything to do with a domain coercion (for apparently good reasons,
given other comments in that file).  However, if we take out the
expression_planner() and replace it with a call to
strip_implicit_coercions(), not only it magically starts working, but
also the regression tests start failing with the attached diff, which
seems a Good Thing to me.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
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