Parallel executor support.

This code provides infrastructure for a parallel leader to start up
parallel workers to execute subtrees of the plan tree being executed
in the master.  User-supplied parameters from ParamListInfo are passed
down, but PARAM_EXEC parameters are not.  Various other constructs,
such as initplans, subplans, and CTEs, are also not currently shared.
Nevertheless, there's enough here to support a basic implementation of
parallel query, and we can lift some of the current restrictions as
needed.

Amit Kapila and Robert Haas

Author: robertmhaas

src/backend/executor/Makefile

@@ -13,7 +13,8 @@ top_builddir = ../../..
 include $(top_builddir)/src/Makefile.global
 
 OBJS = execAmi.o execCurrent.o execGrouping.o execIndexing.o execJunk.o \
-       execMain.o execProcnode.o execQual.o execScan.o execTuples.o \
+       execMain.o execParallel.o execProcnode.o execQual.o \
+       execScan.o execTuples.o \
        execUtils.o functions.o instrument.o nodeAppend.o nodeAgg.o \
        nodeBitmapAnd.o nodeBitmapOr.o \
        nodeBitmapHeapscan.o nodeBitmapIndexscan.o nodeCustom.o nodeHash.o \