Add benchmark of integrated probability

Add benchmark of integrated probability within the union of a set
of fields.

healpix_alchemy/tests/benchmarks/test_benchmarks.py

@@ -104,3 +104,22 @@ def test_fields_in_90pct_credible_region(bench, random_fields, random_sky_map):
 
     # Run benchmark
     bench(query)
+
+
+def test_integrated_probability(bench, random_fields, random_sky_map):
+    """Find integrated probability within union of fields."""
+    union = sa.select(
+        func.union(FieldTile.hpx).label('hpx')
+    ).subquery()
+
+    area = (union.columns.hpx * SkymapTile.hpx).area
+    prob = sa.func.sum(SkymapTile.probdensity * area)
+
+    query = sa.select(
+        prob
+    ).filter(
+        SkymapTile.id == 1,
+        union.columns.hpx.overlaps(SkymapTile.hpx)
+    )
+
+    bench(query)