parallel-workload: Back to smaller expressions

Too many OoMs: https://buildkite.com/materialize/nightly/builds/13962

Author: def-

misc/python/materialize/parallel_workload/expression.py

@@ -277,7 +277,7 @@ def expression(
     level: int = 0,
 ) -> str:
     if level < 60:
-        if FUNC_OPS[data_type] and rng.random() < 0.7:
+        if FUNC_OPS[data_type] and rng.random() < 0.5:
             fnop = rng.choice(FUNC_OPS[data_type])
             if (
                 kind == ExprKind.ALL