ALTER <thing> [IF EXISTS] ... allows silent DDL if required,

doc/src/sgml/ref/alter_foreign_table.sgml

doc/src/sgml/ref/alter_index.sgml

doc/src/sgml/ref/alter_sequence.sgml

doc/src/sgml/ref/alter_table.sgml

doc/src/sgml/ref/alter_view.sgml

src/backend/commands/sequence.c

src/backend/commands/tablecmds.c

src/backend/nodes/copyfuncs.c

src/backend/nodes/equalfuncs.c

src/backend/parser/gram.y

src/backend/parser/parse_utilcmd.c

src/backend/tcop/utility.c

src/include/nodes/parsenodes.h

src/test/regress/expected/alter_table.out

src/test/regress/expected/foreign_data.out

src/test/regress/expected/sequence.out

src/test/regress/sql/alter_table.sql

src/test/regress/sql/foreign_data.sql

src/test/regress/sql/sequence.sql