fe/src/main/java/org/apache/impala/rewrite/ExprRewriteRule.java

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package org.apache.impala.rewrite;

import org.apache.impala.analysis.Analyzer;
import org.apache.impala.analysis.Expr;
import org.apache.impala.common.AnalysisException;

/**
 * Base class for all Expr rewrite rules. A rule is free to modify Exprs in place,
 * but must return a different Expr object if any modifications were made.
 * An ExprRewriteRule is intended to apply its transformation on a single Expr and not
 * recursively on all its children. The recursive and repeated application of
 * ExprRewriteRules is driven by an ExprRewriter.
 * An ExprRewriteRule should preserve the type of the transformed expression to avoid
 * having to re-analyze its parent (if any). This behavior guarantees that the Expr
 * tree remains valid even if arbitrary subexpressions are transformed. It also means
 * that the transformed expression may have a wider type than necessary. Callers are
 * free to reset() and analyze() the result if they desire the minimal type.
 */
public interface ExprRewriteRule {
  /**
   * Applies this rewrite rule to the given analyzed Expr. Returns the transformed and
   * analyzed Expr or the original unmodified Expr if no changes were made. If any
   * changes were made, the transformed Expr is guaranteed to be a different Expr object,
   * so callers can rely on object reference comparison for change detection.
   */
  public abstract Expr apply(Expr expr, Analyzer analyzer) throws AnalysisException;
}