ttl opts

Update ttl-options.md

docs-2.0/3.ngql-guide/8.clauses-and-options/ttl-options.md

@@ -1,6 +1,6 @@
 # TTL
 
-TTL (Time To Live) specifies a timeout for a property. Once timed out, the property expires.
+TTL (Time To Live) is a mechanism in NebulaGraph that defines the lifespan of data. Once the data reaches its predefined lifespan, it is automatically deleted from the database. This feature is particularly suitable for data that only needs temporary storage, such as temporary sessions or cached data.
 
 ## OpenCypher Compatibility
 
@@ -22,7 +22,7 @@ The native nGQL TTL feature has the following options.
 
 |Option|Description|
 |:---|:---|
-|`ttl_col`|Specifies the property to set a timeout on. The data type of the property must be `int` or `timestamp`.|
+|`ttl_col`|Specifies an existing property to set a lifespan on. The data type of the property must be `int` or `timestamp`.|
 |`ttl_duration`|Specifies the timeout adds-on value in seconds. The value must be a non-negative int64 number. A property expires if the sum of its value and the `ttl_duration` value is smaller than the current timestamp. If the `ttl_duration` value is `0`, the property never expires.<br/>You can set `ttl_use_ms` to `true` in the configuration file `nebula-storaged.conf` (default path: `/usr/local/nightly/etc/`) to set the default unit to milliseconds.|
 
 !!! caution
@@ -31,39 +31,13 @@ The native nGQL TTL feature has the following options.
 
     - After setting `ttl_use_ms` to `true`, which sets the default TTL unit to milliseconds, the data type of the property specified by `ttl_col` must be `int`, and the property value needs to be manually converted to milliseconds. For example, when setting `ttl_col` to `a`, you need to convert the value of `a` to milliseconds, such as when the value of `a` is `now()`, you need to set the value of `a` to `now() * 1000`.
 
+## Use TTL options
 
-## Data expiration and deletion
-
-!!! caution
-
-    - When the TTL options are set for a property of a tag or an edge type and the property's value is `NULL`, the property never expires. 
-    - If a property with a default value of `now()` is added to a tag or an edge type and the TTL options are set for the property, the history data related to the tag or the edge type will never expire because the value of that property for the history data is the current timestamp.
-
-### Vertex property expiration
-
-Vertex property expiration has the following impact.
-
-* If a vertex has only one tag, once a property of the vertex expires, the vertex expires.
-
-* If a vertex has multiple tags, once a property of the vertex expires, properties bound to the same tag with the expired property also expire, but the vertex does not expire and other properties of it remain untouched.
-
-### Edge property expiration
-
-Since an edge can have only one edge type, once an edge property expires, the edge expires.
-
-### Data deletion
-
-The expired data are still stored on the disk, but queries will filter them out.
-
-NebulaGraph automatically deletes the expired data and reclaims the disk space during the next [compaction](../../8.service-tuning/compaction.md).
-
-!!! note
-
-    If TTL is [disabled](#remove_a_timeout), the corresponding data deleted after the last compaction can be queried again.
+You must use the TTL options together to set a lifespan on a property.
 
-## Use TTL options
+Before using the TTL feature, you must first create a timestamp or integer property and specify it in the TTL options. NebulaGraph will not automatically create or manage this timestamp property for you.
 
-You must use the TTL options together to set a valid timeout on a property.
+When inserting the value of the timestamp or integer property, it is recommended to use the `now()` function or the current timestamp to represent the present time.
 
 ### Set a timeout if a tag or an edge type exists
 
@@ -91,6 +65,34 @@ nebula> CREATE TAG IF NOT EXISTS t2(a int, b int, c string) TTL_DURATION= 100, T
 # Insert a vertex with tag t2. The timeout timestamp is 1648197238 (1648197138 + 100).
 nebula> INSERT VERTEX t2(a, b, c) VALUES "102":(1648197138, 30, "Hello");
 ```
+## Data expiration and deletion
+
+!!! caution
+
+    - When the TTL options are set for a property of a tag or an edge type and the property's value is `NULL`, the property never expires. 
+    - If a property with a default value of `now()` is added to a tag or an edge type and the TTL options are set for the property, the history data related to the tag or the edge type will never expire because the value of that property for the history data is the current timestamp.
+
+### Vertex property expiration
+
+Vertex property expiration has the following impact.
+
+* If a vertex has only one tag, once a property of the vertex expires, the vertex expires.
+
+* If a vertex has multiple tags, once a property of the vertex expires, properties bound to the same tag with the expired property also expire, but the vertex does not expire and other properties of it remain untouched.
+
+### Edge property expiration
+
+Since an edge can have only one edge type, once an edge property expires, the edge expires.
+
+### Data deletion
+
+The expired data are still stored on the disk, but queries will filter them out.
+
+NebulaGraph automatically deletes the expired data and reclaims the disk space during the next [compaction](../../8.service-tuning/compaction.md).
+
+!!! note
+
+    If TTL is [disabled](#remove_a_timeout), the corresponding data deleted after the last compaction can be queried again.
 
 ## Remove a timeout