calcite如何写UDF

Calcite的UDF可以使用Java编写，需要实现`org.apache.calcite.adapter.enumerable.CallImplementor`接口。以下是一个示例：

import org.apache.calcite.adapter.enumerable.CallImplementor;
import org.apache.calcite.adapter.enumerable.NullPolicy;
import org.apache.calcite.adapter.jdbc.JdbcImplementor;
import org.apache.calcite.adapter.jdbc.JdbcUtils;
import org.apache.calcite.linq4j.tree.BlockBuilder;
import org.apache.calcite.linq4j.tree.Expression;
import org.apache.calcite.linq4j.tree.Expressions;
import org.apache.calcite.linq4j.tree.MethodCallExpression;
import org.apache.calcite.linq4j.tree.TypeSystem;
import org.apache.calcite.rel.type.RelDataType;
import org.apache.calcite.sql.SqlCall;
import org.apache.calcite.sql.SqlFunction;
import org.apache.calcite.sql.SqlKind;
import org.apache.calcite.sql.SqlLiteral;
import org.apache.calcite.sql.SqlNode;
import org.apache.calcite.sql.SqlOperandCountRange;
import org.apache.calcite.sql.SqlOperatorBinding;
import org.apache.calcite.sql.SqlUtil;
import org.apache.calcite.sql.fun.SqlStdOperatorTable;
import org.apache.calcite.sql.type.SqlOperandTypeInference;
import org.apache.calcite.sql.type.SqlOperandTypeInferenceConsistency;
import org.apache.calcite.sql.type.SqlReturnTypeInference;
import org.apache.calcite.sql.type.SqlTypeUtil;
import org.apache.calcite.util.Optionality;

import com.google.common.collect.ImmutableList;

public class MyUdf implements SqlFunction {

    private final String name;

    public MyUdf(String name) {
        this.name = name;
    }

    @Override
    public SqlKind getKind() {
        return SqlKind.OTHER_FUNCTION;
    }

    @Override
    public String getName() {
        return name;
    }

    @Override
    public boolean isDeterministic() {
        return true;
    }

    @Override
    public boolean isDynamicFunction() {
        return false;
    }

    @Override
    public boolean isNiladic() {
        return false;
    }

    @Override
    public SqlOperandCountRange getOperandCountRange() {
        return SqlOperandCountRange.from(1);
    }

    @Override
    public boolean requiresDecimalExpansion() {
        return false;
    }

    @Override
    public Optionality getParamOptional(int i) {
        return Optionality.FORBIDDEN;
    }

    @Override
    public RelDataType inferReturnType(SqlOperatorBinding opBinding) {
        return opBinding.getTypeFactory().createSqlType(TypeSystem.DEFAULT.getTypeFactory().createSqlType(SqlTypeName.INTEGER));
    }

    @Override
    public SqlOperandTypeInference getOperandTypeInference() {
        return SqlOperandTypeInferenceConsistency.LEAST_RESTRICTIVE;
    }

    @Override
    public SqlReturnTypeInference getReturnTypeInference() {
        return opBinding -> inferReturnType(opBinding);
    }

    @Override
    public CallImplementor getImplementor() {
        return new JdbcImplementor() {

            @Override
            public Expression implementCall(JdbcImplementor.Context context, BlockBuilder builder, SqlCall call, NullPolicy nullPolicy) {
                Expression jdbcCall = super.implementCall(context, builder, call, nullPolicy);
                MethodCallExpression callExpression = Expressions.call(jdbcCall, "getInt");
                return callExpression;
            }

            @Override
            public void visitChild(int i, SqlNode operand) {
                if (i == 0) {
                    JdbcUtils.quoteOperand(operand, appendable);
                } else {
                    super.visitChild(i, operand);
                }
            }

        };
    }

    @Override
    public SqlCall createCall(SqlLiteral functionQualifier, SqlParserPos pos, SqlNode... operands) {
        return createCall(operands);
    }

    @Override
    public SqlCall createCall(SqlNode... operands) {
        return new SqlFunctionCall(this, ImmutableList.copyOf(operands), SqlParserPos.ZERO);
    }
}

在上面的代码中，我们实现了一个名为`MyUdf`的UDF，它接受一个整数类型的参数，并返回一个整数类型的结果。

我们需要实现`SqlFunction`接口，并重写其中的方法。其中，`getOperandCountRange`方法指定了UDF的参数个数，`inferReturnType`方法指定了UDF的返回类型，`getImplementor`方法指定了UDF的具体实现。在这个示例中，我们使用了`JdbcImplementor`来生成SQL语句，并将其转换成`MethodCallExpression`，然后返回这个表达式作为UDF的实现。

然后，我们可以将这个UDF注册到Calcite的`SqlOperatorTable`中：

SqlOperatorTable operatorTable = frameworkConfig.getOperatorTable();
operatorTable.register(new MyUdf("MY_UDF"));

现在，我们就可以在SQL语句中使用`MY_UDF`函数了：

SELECT MY_UDF(id) FROM my_table