Default.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.exceptions.AnalysisException;
import org.apache.doris.nereids.trees.expressions.Expression;
import org.apache.doris.nereids.trees.expressions.SlotReference;
import org.apache.doris.nereids.trees.expressions.functions.AlwaysNullable;
import org.apache.doris.nereids.trees.expressions.functions.CustomSignature;
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 com.google.common.base.Preconditions;

import java.util.List;

/**
 * ScalarFunction 'default'. This function returns the default value of a column.
 */
public class Default extends ScalarFunction
        implements UnaryExpression, CustomSignature, AlwaysNullable {

    /**
     * constructor with 1 argument.
     */
    public Default(Expression arg) {
        super("default", arg);
    }

    /** constructor for withChildren and reuse signature */
    private Default(ScalarFunctionParams functionParams) {
        super(functionParams);
    }

    /**
     * withChildren.
     */
    @Override
    public Default withChildren(List<Expression> children) {
        Preconditions.checkArgument(children.size() == 1);
        return new Default(getFunctionParams(children));
    }

    @Override
    public FunctionSignature customSignature() {
        // Return signature that accepts any type and returns the same type (but nullable)
        DataType argType = getArgumentType(0);
        return FunctionSignature.ret(argType).args(argType);
    }

    @Override
    public void checkLegalityAfterRewrite() {
        Expression arg = getArgument(0);
        if (!(arg instanceof SlotReference)) {
            throw new AnalysisException("DEFAULT function requires a column reference, not a constant or expression");
        }
    }

    @Override
    public void checkLegalityBeforeTypeCoercion() {
        checkLegalityAfterRewrite();
    }

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