chore: comment

Signed-off-by: discord9 <discord9@163.com>

src/query/src/datafusion.rs

@@ -478,6 +478,19 @@ impl QueryEngine for DatafusionQueryEngine {
     fn engine_context(&self, query_ctx: QueryContextRef) -> QueryEngineContext {
         let mut state = self.state.session_state();
         state.config_mut().set_extension(query_ctx.clone());
+        // note that hints in "x-greptime-hints" is automatically parsed
+        // and set to query context's extension, so we can get it from query context.
+        if let Some(parallelism) = query_ctx.extension("query_parallelism") {
+            if let Ok(n) = parallelism.parse::<u64>() {
+                let new_cfg = state.config().clone().with_target_partitions(n as usize);
+                *state.config_mut() = new_cfg;
+            } else {
+                common_telemetry::warn!(
+                    "Failed to parse query_parallelism: {}, using default value",
+                    parallelism
+                );
+            }
+        }
         QueryEngineContext::new(state, query_ctx)
     }
 

src/session/src/hints.rs

@@ -14,6 +14,7 @@
 
 // For the given format: `x-greptime-hints: auto_create_table=true, ttl=7d`
 pub const HINTS_KEY: &str = "x-greptime-hints";
+/// Deprecated, use `HINTS_KEY` instead. Notes if "x-greptime-hints" is set, keys with this prefix will be ignored.
 pub const HINTS_KEY_PREFIX: &str = "x-greptime-hint-";
 
 pub const READ_PREFERENCE_HINT: &str = "read_preference";