CaseWhenToIf.java

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package org.apache.doris.nereids.rules.expression.rules;

import org.apache.doris.nereids.rules.expression.ExpressionPatternMatcher;
import org.apache.doris.nereids.rules.expression.ExpressionPatternRuleFactory;
import org.apache.doris.nereids.rules.expression.ExpressionRuleType;
import org.apache.doris.nereids.trees.expressions.CaseWhen;
import org.apache.doris.nereids.trees.expressions.Expression;
import org.apache.doris.nereids.trees.expressions.WhenClause;
import org.apache.doris.nereids.trees.expressions.functions.scalar.If;
import org.apache.doris.nereids.trees.expressions.literal.NullLiteral;

import com.google.common.collect.ImmutableList;

import java.util.List;

/**
 * Rewrite rule to convert CASE WHEN to IF.
 * For example:
 * CASE WHEN a > 1 THEN 1 ELSE 0 END -> IF(a > 1, 1, 0)
 */
public class CaseWhenToIf implements ExpressionPatternRuleFactory {

    public static CaseWhenToIf INSTANCE = new CaseWhenToIf();

    @Override
    public List<ExpressionPatternMatcher<? extends Expression>> buildRules() {
        return ImmutableList.of(
                matchesTopType(CaseWhen.class).then(CaseWhenToIf::rewrite)
                        .toRule(ExpressionRuleType.CASE_WHEN_TO_IF)
        );
    }

    private static Expression rewrite(CaseWhen caseWhen) {
        Expression expr = caseWhen;
        if (caseWhen.getWhenClauses().size() == 1) {
            WhenClause whenClause = caseWhen.getWhenClauses().get(0);
            Expression operand = whenClause.getOperand();
            Expression result = whenClause.getResult();
            expr = new If(operand, result, caseWhen.getDefaultValue().orElse(new NullLiteral(result.getDataType())));
        }
        // TODO: traverse expr in CASE WHEN / If.
        return expr;
    }
}