make batch range scan sequentially

Signed-off-by: Runji Wang <[email protected]>

src/batch/src/executor/row_seq_scan.rs

@@ -33,7 +33,6 @@ use risingwave_storage::store::PrefetchOptions;
 use risingwave_storage::table::batch_table::storage_table::StorageTable;
 use risingwave_storage::table::{collect_data_chunk, TableDistribution};
 use risingwave_storage::{dispatch_state_store, StateStore};
-use rw_futures_util::select_all;
 
 use crate::error::{BatchError, Result};
 use crate::executor::{
@@ -319,28 +318,28 @@ impl<S: StateStore> RowSeqScanExecutor<S> {
         }
 
         // Range Scan
-        let range_scans = select_all(range_scans.into_iter().map(|range_scan| {
-            let table = table.clone();
-            let histogram = histogram.clone();
-            Box::pin(Self::execute_range(
-                table,
-                range_scan,
+        // WARN: DO NOT use `select` to execute range scans concurrently
+        //       it can consume too much memory if there're too many ranges.
+        for range in range_scans {
+            let stream = Self::execute_range(
+                table.clone(),
+                range,
                 ordered,
                 epoch.clone(),
                 chunk_size,
                 limit,
-                histogram,
-            ))
-        }));
-        #[for_await]
-        for chunk in range_scans {
-            let chunk = chunk?;
-            returned += chunk.cardinality() as u64;
-            yield chunk;
-            if let Some(limit) = &limit
-                && returned >= *limit
-            {
-                return Ok(());
+                histogram.clone(),
+            );
+            #[for_await]
+            for chunk in stream {
+                let chunk = chunk?;
+                returned += chunk.cardinality() as u64;
+                yield chunk;
+                if let Some(limit) = &limit
+                    && returned >= *limit
+                {
+                    return Ok(());
+                }
             }
         }
     }