Update distance_functions.cpp

src/gausskernel/dbmind/db4ai/executor/distance_functions.cpp

@@ -37,6 +37,13 @@ IDENTIFICATION
 #endif
 
 
+/*Euclidean distance: the distance between two points,
+that is, the distance we usually calculate.
+Manhattan distance: the sum of absolute
+wheelbase of two points in the standard coordinate system.
+Chebyshev distance: the maximum value of the
+numerical difference of each coordinate.*/
+
 /*
  * L1 distance (Manhattan)
  * We sum using cascaded summation
@@ -44,6 +51,10 @@ IDENTIFICATION
  * are not available or for the the case that the dimension is not a multiple
  * of the width of the registers
  */
+
+/*Vectorization refers to using an array instead of a scalar 
+to manipulate each element in the array.*/
+
 static force_inline double l1_non_vectorized(double const * p, double const * q, uint32_t const dimension)
 {
     double term = 0.;