properties()-function-reduces-query-performance (#1978)

* properties()-function-reduces-query-performance

* Update 4.schema.md

docs-2.0/3.ngql-guide/6.functions-and-expressions/4.schema.md

@@ -51,6 +51,19 @@ nebula> LOOKUP ON player WHERE player.age  > 45 \
 +-------------------------------------+
 ```
 
+You can also use the property reference symbols (`$^` and `$$`) instead of the `vertex` field in the `properties()` function to get all properties of a vertex.
+
+- `$^` represents the data of the starting vertex at the beginning of exploration. For example, in `GO FROM "player100" OVER follow reversely YIELD properties($^)`, `$^` refers to the vertex `player100`.
+
+- `$$` represents the data of the end vertex at the end of exploration.
+
+`properties($^)` and `properties($$)` are generally used in `GO` statements. For more information, see [Property reference](../5.operators/5.property-reference.md).
+
+!!! caution
+
+    You can use `properties().<property_name>` to get a specific property of a vertex. However, it is not recommended to use this method to obtain specific properties because the `properties()` function returns all properties, which can decrease query performance.
+
+
 ### properties(edge)
 
 properties(edge) returns the properties of an edge.
@@ -72,6 +85,10 @@ nebula> GO FROM "player100" OVER follow \
 +------------------+
 ```
 
+!!! caution
+
+    You can use `properties(edge).<property_name>` to get a specific property of an edge. However, it is not recommended to use this method to obtain specific properties because the `properties(edge)` function returns all properties, which can decrease query performance.
+
 ### type(edge)
 
 type(edge) returns the edge type of an edge.