VariantFlatten.java

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package org.apache.doris.nereids.trees.expressions.functions.scalar;

import org.apache.doris.catalog.FunctionSignature;
import org.apache.doris.nereids.trees.expressions.Expression;
import org.apache.doris.nereids.trees.expressions.functions.ExplicitlyCastableSignature;
import org.apache.doris.nereids.trees.expressions.functions.PropagateNullable;
import org.apache.doris.nereids.trees.expressions.shape.UnaryExpression;
import org.apache.doris.nereids.trees.expressions.visitor.ExpressionVisitor;
import org.apache.doris.nereids.types.DataType;
import org.apache.doris.nereids.types.StringType;
import org.apache.doris.nereids.types.VariantType;

import com.google.common.base.Preconditions;
import com.google.common.collect.ImmutableList;

import java.util.List;

/**
 * ScalarFunction 'variant_flatten'. Flattens a nested VARIANT value into a
 * single-level JSON object whose keys are dot-joined paths to each leaf:
 *   {"a":{"b":2}}          -> {"a.b":2}
 *   {"a":{"b":{"c":3}}}    -> {"a.b.c":3}
 *   {"a":[{"b":1}]}        -> {"a":[{"b":1}]}   // arrays are opaque leaves
 * Returns STRING rather than VARIANT so the result is not re-structured into
 * sub-columns when written back to a VARIANT column. Nullability propagates
 * from the input: a NULL variant row yields a NULL string.
 */
public class VariantFlatten extends ScalarFunction
        implements UnaryExpression, ExplicitlyCastableSignature, PropagateNullable {

    public static final List<FunctionSignature> SIGNATURES = ImmutableList.of(
            FunctionSignature.ret(StringType.INSTANCE).args(new VariantType(0))
    );

    public VariantFlatten(Expression arg0) {
        super("variant_flatten", arg0);
    }

    @Override
    public VariantFlatten withChildren(List<Expression> children) {
        Preconditions.checkArgument(children.size() == 1);
        return new VariantFlatten(children.get(0));
    }

    @Override
    public List<FunctionSignature> getSignatures() {
        return SIGNATURES;
    }

    @Override
    public FunctionSignature computeSignature(FunctionSignature signature) {
        List<Expression> arguments = getArguments();
        DataType expressionType = arguments.get(0).getDataType();
        DataType sigType = signature.argumentsTypes.get(0);
        if (expressionType instanceof VariantType && sigType instanceof VariantType) {
            signature = signature.withArgumentType(0, expressionType);
        }
        return super.computeSignature(signature);
    }

    @Override
    public <R, C> R accept(ExpressionVisitor<R, C> visitor, C context) {
        return visitor.visitVariantFlatten(this, context);
    }
}