SERVER-25957 Optimize $indexOfArray when array argument is constant.

Signed-off-by: Charlie Swanson <charlie.swanson@mongodb.com>

Closes #1229

1 files changed, 29 insertions, 2 deletions

diff --git a/src/mongo/db/pipeline/expression.h b/src/mongo/db/pipeline/expression.h
index 5b4a0286b7c..6cfd4603bac 100644
--- a/src/mongo/db/pipeline/expression.h
+++ b/src/mongo/db/pipeline/expression.h
@@ -728,6 +728,7 @@ public:
 
     Value evaluate(const Document& root) const final;
     Value serialize(bool explain) const final;
+    boost::intrusive_ptr<Expression> optimize() final;
     const char* getOpName() const final;
 };
 
@@ -1218,13 +1219,39 @@ public:
 };
 
 
-class ExpressionIndexOfArray final : public ExpressionRangedArity<ExpressionIndexOfArray, 2, 4> {
+class ExpressionIndexOfArray : public ExpressionRangedArity<ExpressionIndexOfArray, 2, 4> {
 public:
     explicit ExpressionIndexOfArray(const boost::intrusive_ptr<ExpressionContext>& expCtx)
         : ExpressionRangedArity<ExpressionIndexOfArray, 2, 4>(expCtx) {}
 
-    Value evaluate(const Document& root) const final;
+
+    Value evaluate(const Document& root) const;
+    boost::intrusive_ptr<Expression> optimize() final;
     const char* getOpName() const final;
+
+protected:
+    struct Arguments {
+        Arguments(Value targetOfSearch, int startIndex, int endIndex)
+            : targetOfSearch(targetOfSearch), startIndex(startIndex), endIndex(endIndex) {}
+
+        Value targetOfSearch;
+        int startIndex;
+        int endIndex;
+    };
+    /**
+     * When given 'operands' which correspond to the arguments to $indexOfArray, evaluates and
+     * validates the target value, starting index, and ending index arguments and returns their
+     * values as a Arguments struct. The starting index and ending index are optional, so as default
+     * 'startIndex' will be 0 and 'endIndex' will be the length of the input array. Throws a
+     * UserException if the values are found to be invalid in some way, e.g. if the indexes are not
+     * numbers.
+     */
+    Arguments evaluateAndValidateArguments(const Document& root,
+                                           const ExpressionVector& operands,
+                                           size_t arrayLength) const;
+
+private:
+    class Optimized;
 };