Share per-chunk JoinLeftData across right partitions in NLJ memory-limited fallback

datafusion/common/src/config.rs

@@ -860,6 +860,25 @@ config_namespace! {
         /// Default: 128 MB
         pub max_spill_file_size_bytes: ConfigNonZeroUsize, default = non_zero_usize_default(128 * 1024 * 1024)
 
+        /// Enables the memory-limited fallback for `NestedLoopJoinExec` join
+        /// types that emit unmatched left rows in the final output (LEFT, LEFT
+        /// SEMI, LEFT ANTI, LEFT MARK, FULL) when the right side has multiple
+        /// partitions.
+        ///
+        /// This fallback coordinates per-chunk left state (visited bitmap and
+        /// probe-thread counter) across all right-side partitions, which
+        /// assumes every partition runs in the same process. Distributed
+        /// engines that execute each output partition as an independent task
+        /// (e.g. Ballista, datafusion-distributed) build a separate coordinator
+        /// per task and poll only one partition, so the cross-partition
+        /// counter never reaches zero and the fallback would stall. Such
+        /// engines should set this to `false`: the coordinated fallback is then
+        /// disabled for left-emitting multi-partition joins, which instead fail
+        /// with a resource-exhaustion error under memory pressure rather than
+        /// deadlocking. Single-partition and non-left-emitting joins are
+        /// unaffected and always keep the fallback.
+        pub enable_nlj_coordinated_fallback: bool, default = true
+
         /// Number of files to read in parallel when inferring schema and statistics
         pub meta_fetch_concurrency: ConfigNonZeroUsize, default = non_zero_usize_default(32)