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

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package org.apache.impala.rewrite;

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

/**
 * This rule replaces a constant Expr with its equivalent LiteralExpr by evaluating the
 * Expr in the BE. Exprs that are already LiteralExprs are not changed.
 *
 * TODO: Expressions fed into this rule are currently not required to be analyzed
 * in order to support constant folding in expressions that contain unresolved
 * references to select-list aliases (such expressions cannot be analyzed).
 * The cross-dependencies between rule transformations and analysis are vague at the
 * moment and make rule application overly complicated.
 *
 * Examples:
 * 1 + 1 + 1 --> 3
 * toupper('abc') --> 'ABC'
 * cast('2016-11-09' as timestamp) --> TIMESTAMP '2016-11-09 00:00:00'
 */
public class FoldConstantsRule implements ExprRewriteRule {
  public static ExprRewriteRule INSTANCE = new FoldConstantsRule();

  @Override
  public Expr apply(Expr expr, Analyzer analyzer) throws AnalysisException {
    // Avoid calling Expr.isConstant() because that would lead to repeated traversals
    // of the Expr tree. Assumes the bottom-up application of this rule. Constant
    // children should have been folded at this point.
    for (Expr child: expr.getChildren()) if (!Expr.IS_LITERAL.apply(child)) return expr;
    if (Expr.IS_LITERAL.apply(expr) || !expr.isConstant()) return expr;

    // Do not constant fold cast(null as dataType) because we cannot preserve the
    // cast-to-types and that can lead to query failures, e.g., CTAS
    if (expr instanceof CastExpr) {
      CastExpr castExpr = (CastExpr) expr;
      if (Expr.IS_NULL_LITERAL.apply(castExpr.getChild(0))) {
        return expr;
      }
    }
    // Analyze constant exprs, if necessary. Note that the 'expr' may become non-constant
    // after analysis (e.g., aggregate functions).
    if (!expr.isAnalyzed()) {
      expr.analyze(analyzer);
      if (!expr.isConstant()) return expr;
    }
    Expr result = LiteralExpr.createBounded(expr, analyzer.getQueryCtx(),
      LiteralExpr.MAX_STRING_LITERAL_SIZE);

    // Preserve original type so parent Exprs do not need to be re-analyzed.
    if (result != null) return result.castTo(expr.getType());
    return expr;
  }

  private FoldConstantsRule() {}
}