Update gist.h

src/include/access/gist.h

@@ -14,6 +14,18 @@
  *
  * -------------------------------------------------------------------------
  */
+
+/*
+Gist(Generalized Search Tree), that is, universal search tree. Like btree, it is also a balanced search tree.
+Different from btree, btree index is often used for operations such as greater than, less than and equal to,
+but in real life, many data are not suitable for this scenario, such as geographic data, images and so on. 
+If we want to query whether there is a certain point in a certain place, that is, to judge the "inclusion" of 
+geographical location, then we can use the gist index. Because the gist index allows you to define rules to 
+distribute any type of data into a balanced tree, and allows you to define a method to use this representation
+for some operators to access. For example, for spatial data, GiST index can use R-tree to support relative position
+operators (left, right, inclusive, etc.), while for tree graph, R-tree can support intersection or inclusion operators.
+*/
+
 #ifndef GIST_H
 #define GIST_H