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'SELECT DISTINCT a,b FROM t1' should not use temp table if there is unique index (or primary key) on a. There are a number of other similar cases that can be calculated without the use of a temp table : multi-part unique indexes, primary keys or using GROUP BY instead of DISTINCT. When a GROUP BY/DISTINCT clause contains all key parts of a unique index, then it is guaranteed that the fields of the clause will be unique, therefore we can optimize away GROUP BY/DISTINCT altogether. This optimization has two effects: * there is no need to create a temporary table to compute the GROUP/DISTINCT operation (or the temporary table will be smaller if only GROUP is removed and DISTINCT stays or if DISTINCT is removed and GROUP BY stays) * this causes the statement in effect to become updatable in Connector/Java because the result set columns will be direct reference to the primary key of the table (instead to the temporary table that it currently references). Implemented a check that will optimize away GROUP BY/DISTINCT for queries like the above. Currently it will work only for single non-constant table in the FROM clause.
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