sqlite perform group by and order by using transient index,
and if there isn't exist such an index, sqlite will create the index and
store it in its a temporary file.
So I think the following SQL statement won't create temporary file in
disk.
create table tbl(a,b,c);
create index on tbl(a);
create index on tbl(b);
SELECT B.a,B.b,B.c FROM
(SELECT a,b FROM tbl GROUP BY a) A
INNER JOIN
(SELECT b,c FROM tbl ORDER BY b)B
ON A.b=B.b
Because both group by and order by could make use of appropriate index .
But in my test, temporary file appear.
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