Power BI custom data connector - how to override cast behavior for int64/uint data type

[Bruce__z]

I'm writing a custom data connector for Click House, to support the direct query feature. The connector is based on Click House ODBC driver.

The connector works for number types except int64 and uint64. When loading the connector in Power BI Desktop and connecting to the database, if I check the int64 field and check a visualization chart, it always show "Can't display the visual". Looking into the details, it says: DB::Exception: Decimal data type family must have exactly two arguments: precision and scale: While processing CAST(i64, 'DECIMAL').

Here is a sample statement - "  select sum(cast(`int64field` as DECIMAL)) as `C1` from `db`.`table`  ".

The reason for above error is that Click House must specify 2 parameters for the Decimal type, as described at Type Conversion | ClickHouse Documentation.

As the sql statement is automatically generated, is there any way to correct the statement while it is generating?

I found seems the cast statement can be modified through "AstVisitor" feature in the sample DataConnectors/RedshiftODBC.pq. However below code doesn't work. (The AstVisitor has already added into OdbcOptions in Odbc.DataSource.)

        AstVisitor = [
            Constant =
                let
                    Cast = (value, typeName) => [
                        Text = Text.Format("CAST(#{0} as #{1})", { value, typeName })
                    ],
                    Visitor = [
                        BIGINT = each Cast(_, "Decimal256(0)")
                    ]
                in
                    (typeInfo, ast) => Record.FieldOrDefault(Visitor, typeInfo[TYPE_NAME], each null)(ast[Value])
        ]

Thank you !

[Anonymous]

HI @Bruce__z,

Thanks for your sharing, I think they will help others who face a similar scenario. 🙂

Regards,

Xiaoxin Sheng

[Bruce__z · Accepted solution]

I didn't find a workaround for modifying the cast statement. But I fixed this issue by modifying the column info in SQLColumns handler. Just change the columns which are int64/uint64 types to DOUBLE type, by updating DATA_TYPE/SQL_DATA_TYPE/TYPE_NAME columns in the metadata table.  This way power BI even doesn't generate the cast statement. Here is a sample for SQLColumns - DataConnectors/SnowflakeODBC.pq at master · microsoft/DataConnectors (github.com)