Presto C++ Limitations

General Limitations

The C++ evaluation engine has a number of limitations:

  • Not all built-in functions are implemented in C++. Attempting to use unimplemented functions results in a query failure. For supported functions, see Function Coverage.

  • Not all built-in types are implemented in C++. Attempting to use unimplemented types will result in a query failure.

    • All basic and structured types in Data Types are supported, except for CHAR, TIME, and TIME WITH TIMEZONE. These are subsumed by VARCHAR, TIMESTAMP and TIMESTAMP WITH TIMEZONE.

    • Presto C++ only supports unlimited length VARCHAR, and does not honor the length n in varchar[n].

    • The following types are not supported: IPADDRESS, IPPREFIX, KHYPERLOGLOG, P4HYPERLOGLOG, QDIGEST, TDIGEST, GEOMETRY, BINGTILE.

  • Certain parts of the plugin SPI are not used by the C++ evaluation engine. In particular, C++ workers will not load any plugin in the plugins directory, and certain plugin types are either partially or completely unsupported.

    • PageSourceProvider, RecordSetProvider, and PageSinkProvider do not work in the C++ evaluation engine.

    • User-supplied functions, types, parametric types and block encodings are not supported.

    • The event listener plugin does not work at the split level.

    • User-defined functions do not work in the same way, see Remote Function Execution.

  • Memory management works differently in the C++ evaluation engine. In particular:

    • The OOM killer is not supported.

    • The reserved pool is not supported.

    • In general, queries may use more memory than they are allowed to through memory arbitration. See Memory Management.

Functions

reduce_agg

In C++ based Presto, reduce_agg is not permitted to return null in either the inputFunction or the combineFunction. In Presto (Java), this is permitted but undefined behavior. For more information about reduce_agg in Presto, see reduce_agg.