Re: [PATCH] Erase the distinctClause if the result is unique by definition

>
> and if we prepare sql outside a transaction, and execute it in the
> transaction, the other session can't drop the constraint until the
> transaction is ended.

And what if you create a view on top of a query containing a distinct clause
rather than using prepared statements?  FWIW your patch doesn't handle such
case at all, without even needing to drop constraints:

CREATE TABLE t (a int primary key, b int not null,  c int);
INSERT INTO t VALUEs(1, 1, 1), (2, 2, 2);
CREATE UNIQUE INDEX t_idx1 on t(b);
CREATE VIEW v1 AS SELECT DISTINCT b FROM t;
EXPLAIN SELECT * FROM v1;
server closed the connection unexpectedly
        This probably means the server terminated abnormally
        before or while processing the request.

I also think this is not the right way to handle this optimization.