batch size

be/src/vec/exec/scan/scanner_scheduler.cpp

@@ -268,8 +268,9 @@ void ScannerScheduler::_scanner_scan(std::shared_ptr<ScannerContext> ctx,
             }
 
             size_t raw_bytes_threshold = config::doris_scanner_row_bytes;
-            size_t raw_bytes_read = 0; bool first_read = true;
-            bool has_limit = scanner->limit() > 0;
+            size_t raw_bytes_read = 0;
+            bool first_read = true;
+            int64_t limit = scanner->limit();
             while (!eos && raw_bytes_read < raw_bytes_threshold) {
                 if (UNLIKELY(ctx->done())) {
                     eos = true;
@@ -317,12 +318,15 @@ void ScannerScheduler::_scanner_scan(std::shared_ptr<ScannerContext> ctx,
                     ctx->inc_block_usage(free_block->allocated_bytes());
                     scan_task->cached_blocks.emplace_back(std::move(free_block), free_block_bytes);
                 }
-                if (has_limit) {
-                    // If this scanner has limit, return immediately and no need to wait raw_bytes_threshold.
+                if (limit < ctx->batch_size()) {
+                    // If this scanner has limit, and less than batch size,
+                    // return immediately and no need to wait raw_bytes_threshold.
                     // This can save time that each scanner may only return a small number of rows,
                     // but rows are enough from all scanners.
                     // If not break, the query like "select * from tbl where id=1 limit 10"
                     // may scan a lot data when the "id=1"'s filter ratio is high.
+                    // If limit is larger than batch size, this rule is skipped,
+                    // to avoid user specify a large limit and causing too much small blocks.
                     break;
                 }
             } // end for while