fix: honor max_row_group_bytes in the parallel Parquet writer

datafusion/common/src/config.rs

@@ -1451,14 +1451,15 @@ config_namespace! {
         /// either limit is reached, whichever comes first.
         pub max_row_group_size: usize, default =  1024 * 1024
 
-        /// (writing) Target maximum size of each row group in bytes. When set,
-        /// the writer flushes whenever either this limit or `max_row_group_size`
-        /// is reached, whichever comes first. Useful for bounding writer memory
-        /// on wide schemas where a row-count limit can map to very different
-        /// byte sizes. Matches the behavior of `parquet.block.size` in
-        /// parquet-mr. If `None` (the default), only the row-count limit
-        /// applies. Currently only honored when `allow_single_file_parallelism`
-        /// is `false`; by default the parallel file writer ignores this limit.
+        /// (writing) Target maximum estimated encoded size of each row group in bytes.
+        /// When set, either this target or `max_row_group_size` triggers a flush. The
+        /// first batch, subject to the row limit, is written before its size can be
+        /// estimated. Subsequent batches are split using the observed average row size,
+        /// so this is not a hard byte or memory limit. The parallel writer synchronizes
+        /// column feedback to match the single-threaded writer's boundaries for the
+        /// same batches. Columns within each slice still encode in parallel, but
+        /// synchronization reduces overlap between slices and may lower write
+        /// throughput. If `None` (the default), only the row-count limit applies.
         pub max_row_group_bytes: Option<MaxRowGroupBytes>, default = None
 
         /// (writing) Sets "created by" property

datafusion/datasource-parquet/Cargo.toml

@@ -63,6 +63,7 @@ criterion = { workspace = true }
 datafusion-functions = { workspace = true }
 datafusion-functions-nested = { workspace = true }
 tempfile = { workspace = true }
+tokio = { workspace = true, features = ["rt-multi-thread"] }
 
 # Note: add additional linter rules in lib.rs.
 # Rust does not support workspace + new linter rules in subcrates yet