[introspection] Set the minimal query parallelism to 2 (#1586)

This commit makes sure that the default parallelism for the query engine
(datafusion) is at least 2. This value translates to 'target partitions'
which is used by datafusion during query execution to control execution
concurrency. Setting this value to 1 however causes #1585, and setting
this to high is unadvised in our case, as datafusion is used for
low-prio introspection tasks.
However once issue #1585 is resolved, we will return to properly sizing
datafusion.

crates/storage-query-datafusion/src/context.rs

@@ -8,6 +8,7 @@
 // the Business Source License, use of this software will be governed
 // by the Apache License, Version 2.0.
 
+use std::cmp::max;
 use std::fmt::Debug;
 use std::sync::Arc;
 
@@ -168,9 +169,14 @@ impl QueryContext {
  // build the session
  //
  let mut session_config = SessionConfig::new();
- if let Some(p) = default_parallelism {
- session_config = session_config.with_target_partitions(p)
- }
+
+ // TODO: this value affects the number of partitions created by datafusion while
+ // executing a query. Setting this to 1 causes the SymmetricHashJoin to fail
+ // which we use for some of the queries.
+ // Resolving this issue is tracked at https:/restatedev/restate/issues/1585
+ let parallelism = default_parallelism.unwrap_or(2);
+ session_config = session_config.with_target_partitions(max(2, parallelism));
+
  session_config = session_config
  .with_allow_symmetric_joins_without_pruning(true)
  .with_information_schema(true)