Read-only transactions, as defined in SQL.

doc/src/sgml/features.sgml

doc/src/sgml/ref/set_transaction.sgml

doc/src/sgml/ref/start_transaction.sgml

doc/src/sgml/release.sgml

src/backend/access/transam/xact.c

src/backend/catalog/namespace.c

src/backend/commands/copy.c

src/backend/executor/execMain.c

src/backend/parser/gram.y

src/backend/tcop/utility.c

src/backend/utils/misc/guc.c

src/bin/psql/tab-complete.c

src/include/access/xact.h

src/include/catalog/namespace.h

src/test/regress/expected/transactions.out

src/test/regress/sql/transactions.sql