Hello.
If you don't have an index on GROUP BY column, you need a lot of memory for
such queries in H2.
You can use the EXPLAIN command to check whether optimization is used or
not.
create table result(id bigint, name varchar, phone int);
-- Without optimization
explain select id, sum(phone) from result group by id;
> SELECT
> "ID",
> SUM("PHONE")
> FROM "PUBLIC"."RESULT"
> /* PUBLIC.RESULT.tableScan */
> GROUP BY "ID"
create index i on result(id);
-- With optimization
explain select id, sum(phone) from result group by id;
> SELECT
> "ID",
> SUM("PHONE")
> FROM "PUBLIC"."RESULT"
> /* PUBLIC.I */
> GROUP BY "ID"
> /* group sorted */
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