MergeSort was sometimes called mergejoin and was confusing. Now

src/backend/commands/explain.c

src/backend/nodes/copyfuncs.c

src/backend/nodes/equalfuncs.c

src/backend/nodes/outfuncs.c

src/backend/nodes/readfuncs.c

src/backend/optimizer/geqo/geqo_eval.c

src/backend/optimizer/path/costsize.c

src/backend/optimizer/path/indxpath.c

src/backend/optimizer/path/joinpath.c

src/backend/optimizer/path/joinrels.c

src/backend/optimizer/path/mergeutils.c

src/backend/optimizer/path/xfunc.c

src/backend/optimizer/plan/createplan.c

src/backend/optimizer/plan/initsplan.c

src/backend/optimizer/util/joininfo.c

src/backend/optimizer/util/pathnode.c

src/backend/tcop/postgres.c

src/backend/utils/cache/lsyscache.c

src/include/nodes/plannodes.h

src/include/nodes/relation.h