Troubleshoot issues with the multi-stage query engine (v2)

Troubleshoot issues with the multi-stage query engine (v2).

Learn how to troubleshoot errors when using the multi-stage query engine (v2), and see multi-stage query engine limitations.

Find instructions on how to enable the multi-stage query engine, or see a high-level overview of how the multi-stage query engine works.

Limitations of the multi-stage query engine

We are continuously improving the v2 multi-stage query engine. A few limitations to call out:

Support for multi value columns is limited

Support for multi value columns is limited to projections, and predicates must use the arrayToMv function. For example, to successfully run the following query:

SELECT count(*), RandomAirports FROM airlineStats GROUP BY RandomAirports

You must include arrayToMv in the query as follows:

SELECT count(*), arrayToMv(RandomAirports) FROM airlineStats GROUP BY arrayToMv(RandomAirports)

Schema and other prefixes are not supported

Schema and other prefixes are not supported in queries. For example, the following queries are not supported:

SELECT* from default.myTable;
SELECT * from schemaName.myTable;

Queries without prefixes are supported:

SELECT * from myTable;

Modifying query behavior based on the cluster config is not supported

Modifying query behavior based on the cluster configuration is not supported. distinctcounthll, distinctcounthllmv, distinctcountrawhll, and `distinctcountrawhllmv` use a different default value of log2mParam in the multi-stage v2 engine. In v2, this value can no longer be configured. Therefore, the following query may produce different results in v1 and v2 engine:

select distinctcounthll(col) from myTable

To ensure v2 returns the same result, specify the log2mParam value in your query:

select distinctcounthll(col, 8) from myTable

Ambiguous reference to a projected column in statement clauses

If a column is repeated more than once in SELECT statement, that column requires disambiguate aliasing. For example, in the following query, the reference to colA is ambiguous whether it's to the first or second projected colA:

SELECT colA, colA, COUNT(*)
FROM myTable GROUP BY colA ORDER BY colA

The solution is to rewrite the query either use aliasing:

SELECT colA AS tmpA, colA as tmpB, COUNT(*) 
FROM myTable GROUP BY tmpA, tmpB ORDER BY tmpA

Or use index-based referencing:

SELECT colA, colA, COUNT(*) 
FROM myTable GROUP BY 1, 2 ORDER BY 1

Tightened restriction on function naming

Pinot single-stage query engine automatically removes the underscore _ character from function names. So co_u_n_t()is equivalent to count().

In v2, function naming restrictions were tightened, so the underscore(_) character is only allowed to separate word boundaries in a function name. Also camel case is supported in function names. For example, the following function names are allowed:

is_distinct_from(...)
isDistinctFrom(...)

Default names for projections with function calls

Default names for projections with function calls are different between v1 and v2.

• For example, in v1, the following query:

  SELECT count(*) from mytable 

Returns the following result:

    "columnNames": [
        "EXPR$0"
      ],

• In v2, the following function:

  SELECT count(*) from mytable

Returns the following result:

      "columnNames": [
        "count(*)"
      ],

Table names and column names are case sensitive

In v2, table and column names and are case sensitive. In v1 they were not. For example, the following two queries are not equivalent in v2:

select * from myTable

select * from mytable

Note: Function names are not case sensitive in v2 or v1.

Arbitrary number of arguments isn't supported

An arbitrary number of arguments is no longer supported in v2. For example, in v1, the following query worked:

select add(1,2,3,4,5) from table

In v2, this query must be rewritten as follows:

select add(1, add(2,add(3, add(4,5)))) from table

NULL function support

• IS NULL and IS NOT NULL functions do not work correctly in v2

• Using the COUNT function on a NULL column does not work correctly in v2

Troubleshoot errors

Troubleshoot semantic/runtime errors and timeout errors.

Semantic/runtime errors

• Try downloading the latest docker image or building from the latest master commit.

  • We continuously push bug fixes for the multi-stage engine so bugs you encountered might have already been fixed in the latest master build.

• Try rewriting your query.

  • Some functions previously supported in the single-stage query engine (v1) may have a new way to express in the multi-stage engine (v2). Check and see if you are using any non-standard SQL functions or semantics.

Timeout errors

• Try reducing the size of the table(s) used.

  • Add higher selectivity filters to the tables.

• Try executing part of the subquery or a simplified version of the query first.

  • This helps to determine the selectivity and scale of the query being executed.

• Try adding more servers.

  • The new multi-stage engine runs distributed across the entire cluster, so adding more servers to partitioned queries such as GROUP BY aggregates, and equality JOINs help speed up the query runtime.