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https://issues.apache.org/jira/browse/DRILL-7931?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17365749#comment-17365749
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ASF GitHub Bot commented on DRILL-7931:
---------------------------------------
paul-rogers edited a comment on pull request #2259:
URL: https://github.com/apache/drill/pull/2259#issuecomment-864223771
Thanks for finding and fixing this bug. It is a very difficult one. This
area of code is complex. It *might* be that the current fix is correct, but I
do suspect that there is a deeper bug than this fix would suggest, so we should
dig in a bit more.
The description suggests that the code is attempting to combine the sum
operation for `stddev()` with the sum operation for `sum()`. I believe that
doing so is a bug unless there was some specific optimization attempt to share
common expressions. Normally the planner would perform this kind of
optimization. The trick, for Drill, is that the planner does not know the data
type, so it must be the runtime that handles such details. I rather doubt that
the original developers had gotten to the level of optimization in code
generation where they attempted to share subexpressions.
As a result, it is a bug if the `sum()` operation for one expression is
combined with that for another column. We can check, what happens with:
```sql
SELECT sum(c) AS a, sum(c) AS b FROM ...
```
Even though the same expression appears twice, and a "normal" planner might
combine them, Drill probably will not. The runtime should compute two sums in
this case, not somehow try to combine them into one. The same is true for your
case
```sql
SELECT sum(c) AS a, stddev(c) AS b FROM ...
```
Then there is the issue of the nullable `bigint` data type. According to the
[SQL Server
docs](https://docs.microsoft.com/en-us/sql/t-sql/functions/stdev-transact-sql?view=sql-server-ver15),
`stddev` will ignore `NULL` values and will return `NULL` only if no
non-`NULL` rows are present. Probably other engines work the same way. The
[Drill docs](http://drill.apache.org/docs/aggregate-and-aggregate-statistical/)
do not say how `NULL`s are handled, but the original developers mostly followed
[Postgres](https://www.postgresql.org/docs/9.1/functions-aggregate.html), among
others. Postgres also uses the "ignore NULL" behavior.
Given this, there should *never* be a nullable anything in the accumulators,
even if the input type *is* nullable. Instead, to implement the "`NULL` if no
values" behavior, the "finalize" step (the one that computes the final output
value), should return a nullable value if the count is zero. Note that Postgres
returns `NULL` for `sum()` when there are no rows instead of 0. This is odd,
I'm not sure if that is what Drill does. (All these details should be in the
Drill docs, but are not. If we figure out how this works in Drill, we should
file a Doc. JIRA ticket to get the information added.)
Next, consider how the accumulators are created. Remember that Drill does
not know the data type until the first row arrives. At that point, Drill has a
data type and can set up the needed accumulators and generate the code for the
operations. My understanding is that the accumulators and code will remain in
place for the rest of the query -- unless the operation receives an
`OK_NEW_SCHEMA` (schema change) from the upstream operator. At that point,
Drill is supposed to handle the change. (A `sum(c)` first saw `c` as `INT`, but
later saw it as `DOUBLE`, say.) I doubt if this actually works: we'd have to
somehow compute the common type of the old and new types, convert the
accumulators, etc. Maybe it works, but I'd not bet on it. (Most schema change
events are not handled in most operators -- they are an intractable problem for
which there is no good *a priori* solution. It is an ongoing embarrassment that
Drill promises to do things which it cannot do, even in principle.)
Let's look at the error message to see what that tells us. Query:
```sql
select col1, stddev(col2), SUM(col2)
FROM (values ('UA', 3), ('USA', 2), ('UA', 3), ('USA', 5), ('USA', 1),
('UA', 9)) x(col1, col2)
GROUP BY col1;
```
Since the data is from a `VALUES` clause, the schema does not change, so we
can rule out that as a possible cause. Instead, the problem is probably a bug
within Drill itself.
```text
rowtype of new rel:
RecordType(CHAR(3) NOT NULL col1, BIGINT $f1, BIGINT $f2, BIGINT NOT NULL
$f3, BIGINT $f4) NOT NULL
rowtype of set:
RecordType(CHAR(3) NOT NULL col1, BIGINT $f1, BIGINT $f2, BIGINT NOT NULL
$f3, BIGINT NOT NULL $f4) NOT NULL
```
We see that the `$f1` is not a very descriptive name. It probably means
"function 1". The pull request description suggests that:
* `$f1` is `STDDEV.sum(col2)`
* `$f2` is `STDDEV.sum(col2*col2)`
* `$f3` is `STDDEV.count(col2)`
* `$f4` is `SUM(col2)`
Where `STDDEV.sum` is something I just made up to mean "the `sum()` function
that implements `STDDEV()`". We can immediately notice some issues:
* *None* of the functions should be nullable, all should be `NOT NULL`
because of the way that SQL handles `NULL`s: they are ignored, so we only
accumulate non-null values. The `NULL`ness will be added to the output column.
* So, the "new rel" is less correct than the "rowtype of set", even though
both are wrong.
So, where does this leave us? It says that we should check:
* Why did code generation decide to create a nullable accumulator? According
to the rules of SQL, discussed above, the accumulators should all be
non-nullable.
* How does the code find the accumulator? Each should have a unique name,
derived, somehow, from the input expression. The `$f1` symbols are internal
names. They clearly do not identify the function in any detail. If we use them
as a key, we have to ensure that we do the same mapping from expression to
`$fn` each time. Otherwise, the names should be more unique such as (making
something up) `$STDDEV$SUM$C`. The question is: are the names getting
confused, or is the problem elsewhere?
Again, thanks for looking into this; this is a very complex part of the
code, so we have to work carefully.
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> Rowtype mismatch in DrillReduceAggregatesRule
> ---------------------------------------------
>
> Key: DRILL-7931
> URL: https://issues.apache.org/jira/browse/DRILL-7931
> Project: Apache Drill
> Issue Type: Bug
> Components: Query Planning & Optimization
> Affects Versions: 1.18.0
> Reporter: wtf
> Assignee: wtf
> Priority: Major
>
> It's work correct for example in case when there is only one aggregation:
> select col1, stddev(col2) FROM(values ('UA', 3), ('USA', 2), ('UA', 3),
> ('USA', 5), ('USA', 1), ('UA', 9)) x(col1, col2) GROUP BY col1 LIMIT 6;
> Or when it's after other aggregations:
> select col1, SUM(col2), stddev(col2) FROM (values ('UA', 3), ('USA', 2),
> ('UA', 3), ('USA', 5), ('USA', 1), ('UA', 9)) x(col1, col2) GROUP BY col1
> LIMIT 6;
> But when we try to put it before an aggregation, like SUM:
> select col1, stddev(col2), SUM(col2) FROM (values ('UA', 3), ('USA', 2),
> ('UA', 3), ('USA', 5), ('USA', 1), ('UA', 9)) x(col1, col2) GROUP BY col1
> LIMIT 6;
> It's failed with error:
> SYSTEM ERROR: AssertionError: Type mismatch:
> rowtype of new rel:
> RecordType(CHAR(3) NOT NULL col1, BIGINT $f1, BIGINT $f2, BIGINT NOT NULL
> $f3, BIGINT $f4) NOT NULL
> rowtype of set:
> RecordType(CHAR(3) NOT NULL col1, BIGINT $f1, BIGINT $f2, BIGINT NOT NULL
> $f3, BIGINT NOT NULL $f4) NOT NULL
> Same for stddev_samp
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