do not expose vnode_count term to user, use max_parallelism instead

Signed-off-by: Bugen Zhao <[email protected]>

src/common/src/session_config/mod.rs

@@ -140,8 +140,11 @@ pub struct SessionConfig {
     #[parameter(default = "UTC", check_hook = check_timezone)]
     timezone: String,
 
-    /// If `STREAMING_PARALLELISM` is non-zero, CREATE MATERIALIZED VIEW/TABLE/INDEX will use it as
-    /// streaming parallelism.
+    /// The execution parallelism for streaming queries, including tables, materialized views, indexes,
+    /// and sinks. Defaults to 0, which means they will be scheduled adaptively based on the cluster size.
+    ///
+    /// If a non-zero value is set, streaming queries will be scheduled to use a fixed number of parallelism.
+    /// Note that the value will be bounded at `STREAMING_MAX_PARALLELISM`.
     #[serde_as(as = "DisplayFromStr")]
     #[parameter(default = ConfigNonZeroU64::default())]
     streaming_parallelism: ConfigNonZeroU64,
@@ -300,8 +303,16 @@ pub struct SessionConfig {
     #[parameter(default = false)]
     bypass_cluster_limits: bool,
 
+    /// The maximum number of parallelism a streaming query can use. Defaults to 256.
+    ///
+    /// Compared to `STREAMING_PARALLELISM`, which configures the initial parallelism, this configures
+    /// the maximum parallelism a streaming query can use in the future, if the cluster size changes or
+    /// users manually change the parallelism with `ALTER .. SET PARALLELISM`.
+    ///
+    /// It's not always a good idea to set this to a very large number, as it may cause performance
+    /// degradation when performing range scans on the table or the materialized view.
     #[parameter(default = VirtualNode::COUNT_FOR_COMPAT, check_hook = check_vnode_count)]
-    vnode_count: usize,
+    streaming_max_parallelism: usize,
 }
 
 fn check_timezone(val: &str) -> Result<(), String> {

src/frontend/src/stream_fragmenter/mod.rs

@@ -144,7 +144,7 @@ pub fn build_graph(plan_node: PlanRef) -> SchedulerResult<StreamFragmentGraphPro
                 .map(|parallelism| Parallelism {
                     parallelism: parallelism.get(),
                 });
-        fragment_graph.expected_vnode_count = config.vnode_count() as _;
+        fragment_graph.expected_vnode_count = config.streaming_max_parallelism() as _;
     }
 
     // Set timezone.

src/meta/src/rpc/ddl_controller.rs

@@ -1634,7 +1634,11 @@ impl DdlController {
         let parallelism = self.resolve_stream_parallelism(specified_parallelism, &cluster_info)?;
         let parallelism_limited = parallelism > max_parallelism;
         if parallelism_limited {
-            tracing::warn!("Too many parallelism, use {} instead", max_parallelism);
+            // TODO(var-vnode): may return error here?
+            tracing::warn!(
+                "Too many parallelism, use max parallelism {} instead",
+                max_parallelism
+            );
         }
         let parallelism = parallelism.min(max_parallelism);