/usr/include/postgresql/10/server/optimizer/ is in postgresql-server-dev-10 10.3-1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/postgresql/10/server/optimizer/clauses.h
/usr/include/postgresql/10/server/optimizer/cost.h
/usr/include/postgresql/10/server/optimizer/geqo.h
/usr/include/postgresql/10/server/optimizer/geqo_copy.h
/usr/include/postgresql/10/server/optimizer/geqo_gene.h
/usr/include/postgresql/10/server/optimizer/geqo_misc.h
/usr/include/postgresql/10/server/optimizer/geqo_mutation.h
/usr/include/postgresql/10/server/optimizer/geqo_pool.h
/usr/include/postgresql/10/server/optimizer/geqo_random.h
/usr/include/postgresql/10/server/optimizer/geqo_recombination.h
/usr/include/postgresql/10/server/optimizer/geqo_selection.h
/usr/include/postgresql/10/server/optimizer/joininfo.h
/usr/include/postgresql/10/server/optimizer/orclauses.h
/usr/include/postgresql/10/server/optimizer/pathnode.h
/usr/include/postgresql/10/server/optimizer/paths.h
/usr/include/postgresql/10/server/optimizer/placeholder.h
/usr/include/postgresql/10/server/optimizer/plancat.h
/usr/include/postgresql/10/server/optimizer/planmain.h
/usr/include/postgresql/10/server/optimizer/planner.h
/usr/include/postgresql/10/server/optimizer/predtest.h
/usr/include/postgresql/10/server/optimizer/prep.h
/usr/include/postgresql/10/server/optimizer/restrictinfo.h
/usr/include/postgresql/10/server/optimizer/subselect.h
/usr/include/postgresql/10/server/optimizer/tlist.h
/usr/include/postgresql/10/server/optimizer/var.h