Allow SET to not start a transaction

OK, I have finished the patch and applied it, with doc updates.

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Bruce Momjian wrote:

We have on the open items list:

Make SET not start a transaction with autocommit off, document it

The attached patch does exactly that (though without documentation). It
checks for the TRANS_DEFAULT transaction state, which is the state right
after a COMMIT. If the backend is in that state, and autocommit is off,
and the command is SET/SHOW/RESET, then a COMMIT will be forced, rather
than keeping the transaction open for the next command. It seems to
work, but i want to test it more before applying to CVS.

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test=> set autocommit = off;
SET
test=> commit;
WARNING: COMMIT: no transaction in progress
COMMIT
test=> set statement_timeout = 9999999;
SET
test=> select 1;
?column?
----------
1
(1 row)

test=> abort;
ROLLBACK
test=> show statement_timeout;
statement_timeout
-------------------
9999999
(1 row)

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