update batch size in session config

current: 1000000
update to 8192 (default value)

this helps in query performance as larger batch size consumes more memory
and slows down performance

with this change, parseable's clickbench numbers are [181,68,68]

Author: nikhilsinhaparseable

src/query/mod.rs

@@ -111,7 +111,7 @@ impl Query {
         let mut config = SessionConfig::default()
             .with_parquet_pruning(true)
             .with_prefer_existing_sort(true)
-            .with_batch_size(1000000);
+            .with_batch_size(20000);
 
         // Pushdown filters allows DF to push the filters as far down in the plan as possible
         // and thus, reducing the number of rows decoded