Consider the following example:
CREATE TABLE test(p DateTime, k int) ENGINE MergeTree PARTITION BY toDate(p) ORDER BY k;
INSERT INTO test VALUES ('2020-09-01 00:01:02', 1), ('2020-09-01 20:01:03', 2), ('2020-09-02 00:01:03', 3);
- SELECT count() FROM test WHERE toDate(p) >= '2020-09-01' AND p <= '2020-09-01 00:00:00'
In this case rpn will be (FUNCTION_IN_RANGE, FUNCTION_UNKNOWN (due to strict), FUNCTION_AND)
and for optimize_trivial_count_query we cannot use index if there is at least one FUNCTION_UNKNOWN.
since there is no post processing and return count() based on only the first predicate is wrong.
Before this patch FUNCTION_UNKNOWN was allowed for optimize_trivial_count_query, and the result was wrong.
And two examples above just to show the difference, the behaviour hadn't been changed with this patch:
- SELECT * FROM test WHERE toDate(p) >= '2020-09-01' AND p <= '2020-09-01 00:00:00'
In this case will be (FUNCTION_IN_RANGE, FUNCTION_IN_RANGE (due to non-strict), FUNCTION_AND)
so it will prune everything out and nothing will be read.
- SELECT * FROM test WHERE toDate(p) >= '2020-09-01' AND toUnixTimestamp(p)%5==0
In this case will be (FUNCTION_IN_RANGE, FUNCTION_UNKNOWN, FUNCTION_AND)
and all, two, partitions will be scanned, but due to filtering later none of rows will be matched.
For now uuids are not generated at all, they are present only if the
part is updated manually (as you can see in the integration test).
The only place where they can be seen today by an end user is in
`system.parts` table. I was looking for hiding this column behind an
option but couldn't find an easy way to do that.
Likely this is also required for WAL, but need to think how not to break
compatibility.
Relates to #13574, https://github.com/ClickHouse/ClickHouse/issues/13574
Next 1: In the upcoming PR the plan is to integrate de-duplication based on
these fingerprints in the query pipeline.
Next 2: We'll enable automatic generation of uuids and come up with a
way for conditionally sending uuids when processing distributed queries
only when part movement is in progress.
