EliminateGroupByConstant.java

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

import org.apache.doris.nereids.rules.Rule;
import org.apache.doris.nereids.rules.RuleType;
import org.apache.doris.nereids.rules.expression.ExpressionRewriteContext;
import org.apache.doris.nereids.rules.expression.rules.FoldConstantRule;
import org.apache.doris.nereids.rules.rewrite.OneRewriteRuleFactory;
import org.apache.doris.nereids.trees.expressions.Expression;
import org.apache.doris.nereids.trees.expressions.NamedExpression;
import org.apache.doris.nereids.trees.plans.Plan;
import org.apache.doris.nereids.trees.plans.logical.LogicalAggregate;
import org.apache.doris.nereids.util.ExpressionUtils;
import org.apache.doris.nereids.util.Utils;

import com.google.common.collect.Sets;

import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Set;

/**
 * eliminate group by constant, like:
 * select 1, 'str', count(*) from t group by t.id, 1, 'str', 3, 2;
 * transform to:
 * select 1, 'str', count(*) from t group by t.id.
 * we are ensured before:
 * 1. aggregation node output contains all the expressions of group by expressions.
 * 2. others are aggregation functions.
 * so we can do the rule by:
 * 1. eliminate all the literal expression that are not integerLiteral of group by expressions.
 * 2. check whether the left literal expressions are in range and replace them to slots.
 */
public class EliminateGroupByConstant extends OneRewriteRuleFactory {
    @Override
    public Rule build() {
        return logicalAggregate().thenApply(ctx -> {
            LogicalAggregate<Plan> aggregate = ctx.root;
            ExpressionRewriteContext context = new ExpressionRewriteContext(ctx.cascadesContext);
            List<Expression> groupByExprs = aggregate.getGroupByExpressions();
            List<NamedExpression> outputExprs = aggregate.getOutputExpressions();
            Set<Expression> slotGroupByExprs = Sets.newLinkedHashSet();
            Map<Expression, Expression> constantExprsReplaceMap = new HashMap<>(groupByExprs.size());
            Expression lit = null;
            for (Expression expression : groupByExprs) {
                // NOTICE: we should not use the expression after fold as new aggregate's output or group expr
                //   because we rely on expression matching to replace subtree that same as group by expr in output
                //   if we do constant folding before normalize aggregate, the subtree will change and matching fail
                //   such as: select a + 1 + 2 + 3, sum(b) from t group by a + 1 + 2
                Expression foldExpression = FoldConstantRule.evaluate(expression, context);
                if (!foldExpression.isConstant()) {
                    slotGroupByExprs.add(expression);
                } else {
                    constantExprsReplaceMap.put(expression, foldExpression);
                    lit = expression;
                }
            }
            if (slotGroupByExprs.isEmpty() && lit != null) {
                slotGroupByExprs.add(lit);
            }
            return aggregate.withGroupByAndOutput(
                    ExpressionUtils.replace(Utils.fastToImmutableList(slotGroupByExprs), constantExprsReplaceMap),
                    ExpressionUtils.replaceNamedExpressions(outputExprs, constantExprsReplaceMap));
        }).toRule(RuleType.ELIMINATE_GROUP_BY_CONSTANT);
    }
}