Merge input cubes only once when computing lazy multimodel statistics (…

…#2518)

Co-authored-by: Valeriu Predoi <valeriu.predoi@gmail.com>

esmvalcore/preprocessor/_multimodel.py

@@ -484,7 +484,8 @@ def _compute_eager(
             input_slices = cubes  # scalar cubes
         else:
             input_slices = [cube[chunk] for cube in cubes]
-        result_slice = _compute(input_slices, operator=operator, **kwargs)
+        combined_cube = _combine(input_slices)
+        result_slice = _compute(combined_cube, operator=operator, **kwargs)
         result_slices.append(result_slice)
 
     try:
@@ -503,10 +504,13 @@ def _compute_eager(
     return result_cube
 
 
-def _compute(cubes: list, *, operator: iris.analysis.Aggregator, **kwargs):
+def _compute(
+    cube: iris.cube.Cube,
+    *,
+    operator: iris.analysis.Aggregator,
+    **kwargs,
+):
     """Compute statistic."""
-    cube = _combine(cubes)
-
     with warnings.catch_warnings():
         warnings.filterwarnings(
             "ignore",
@@ -531,8 +535,6 @@ def _compute(cubes: list, *, operator: iris.analysis.Aggregator, **kwargs):
 
     # Remove concatenation dimension added by _combine
     result_cube.remove_coord(CONCAT_DIM)
-    for cube in cubes:
-        cube.remove_coord(CONCAT_DIM)
 
     # some iris aggregators modify dtype, see e.g.
     # https://numpy.org/doc/stable/reference/generated/numpy.ma.average.html
@@ -545,7 +547,6 @@ def _compute(cubes: list, *, operator: iris.analysis.Aggregator, **kwargs):
             method=cell_method.method,
             coords=cell_method.coord_names,
             intervals=cell_method.intervals,
-            comments=f"input_cubes: {len(cubes)}",
         )
         result_cube.add_cell_method(updated_method)
     return result_cube
@@ -602,27 +603,26 @@ def _multicube_statistics(
     # Calculate statistics
     statistics_cubes = {}
     lazy_input = any(cube.has_lazy_data() for cube in cubes)
-    for stat in statistics:
-        (stat_id, result_cube) = _compute_statistic(cubes, lazy_input, stat)
+    combined_cube = None
+    for statistic in statistics:
+        stat_id = _get_stat_identifier(statistic)
+        logger.debug("Multicube statistics: computing: %s", stat_id)
+
+        (operator, kwargs) = _get_operator_and_kwargs(statistic)
+        (agg, agg_kwargs) = get_iris_aggregator(operator, **kwargs)
+        if lazy_input and agg.lazy_func is not None:
+            if combined_cube is None:
+                # Merge input cubes only once as this is can be computationally
+                # expensive.
+                combined_cube = _combine(cubes)
+            result_cube = _compute(combined_cube, operator=agg, **agg_kwargs)
+        else:
+            result_cube = _compute_eager(cubes, operator=agg, **agg_kwargs)
         statistics_cubes[stat_id] = result_cube
 
     return statistics_cubes
 
 
-def _compute_statistic(cubes, lazy_input, statistic):
-    """Compute a single statistic."""
-    stat_id = _get_stat_identifier(statistic)
-    logger.debug("Multicube statistics: computing: %s", stat_id)
-
-    (operator, kwargs) = _get_operator_and_kwargs(statistic)
-    (agg, agg_kwargs) = get_iris_aggregator(operator, **kwargs)
-    if lazy_input and agg.lazy_func is not None:
-        result_cube = _compute(cubes, operator=agg, **agg_kwargs)
-    else:
-        result_cube = _compute_eager(cubes, operator=agg, **agg_kwargs)
-    return (stat_id, result_cube)
-
-
 def _multiproduct_statistics(
     products,
     statistics,