[GitHub] [drill] paul-rogers commented on pull request #2259: DRILL-7931: Rowtype mismatch in DrillReduceAggregatesRule

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paul-rogers commented 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.
   
   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.)
   
   So, where does this leave us? It says that we should check:
   
   * Does this query do a schema change? If so, then you're probably trying to use code which was not tested and may not work.
   * Otherwise, 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. You mentioned `$1`, etc. These are internal names that usually have more to them. We would expect that `sum(c)` should have some unique name such as `$SUM$C` (not the real name, I just made that up) while the internal `sum()` for `stddev(c)` should have a distinct, different name such as `$STDDEV$SUM$C` (again, I just made that up.) The question is: how are these names assigned and is that code somehow creating duplicate names for the two cases?
   
   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.
   
   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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