MergeProjects.java

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

import org.apache.doris.nereids.rules.Rule;
import org.apache.doris.nereids.rules.RuleType;
import org.apache.doris.nereids.trees.expressions.NamedExpression;
import org.apache.doris.nereids.trees.plans.Plan;
import org.apache.doris.nereids.trees.plans.logical.LogicalProject;

import java.util.List;

/**
 * this rule aims to merge consecutive project. For example:
 * <pre>
 *   project(a)
 *       |
 *   project(a,b)    ->    project(a)
 *       |
 *   project(a, b, c)
 * </pre>
 */
public class MergeProjects extends OneRewriteRuleFactory {

    @Override
    public Rule build() {
        // TODO modify ExtractAndNormalizeWindowExpression to handle nested window functions
        // here we just don't merge two projects if there is any window function
        return logicalProject(logicalProject())
                .when(project -> project.canMergeProjections(project.child()))
                .then(MergeProjects::mergeProjects).toRule(RuleType.MERGE_PROJECTS);
    }

    /** merge projects */
    public static Plan mergeProjects(LogicalProject<?> project) {
        LogicalProject<? extends Plan> childProject = (LogicalProject<?>) project.child();
        List<NamedExpression> projectExpressions = project.mergeProjections(childProject);
        return project.withProjectsAndChild(projectExpressions, childProject.child(0));
    }
}