Adding compatibility documentation. Simplify field name

Author: yongchul

proto/substrait/algebra.proto

@@ -375,10 +375,8 @@ message AggregateRel {
 
   // Various modes of operations of AggregateRel to capture different behaviors across systems.
   message Compatibility {
-    // If true, the AggregateRel must not yield a row on empty input
-    // when specified with non-empty groupings field even when groupings includes
-    // empty grouping sets.
-    bool groupings_yield_no_rows_on_empty_input = 1;
+    // If true, the AggregateRel must not yield a row on empty input.
+    bool yield_no_rows_on_empty_input = 1;
   }
 }
 

site/docs/relations/logical_relations.md

@@ -407,6 +407,25 @@ If at least one grouping expression is present, the aggregation is allowed to no
 | Per Grouping Set | A list of expression grouping that the aggregation measured should be calculated for. | Optional.      |
 | Measures         | A list of one or more aggregate expressions along with an optional filter. | Optional, required if no grouping sets. |
 
+### Aggregate Compatibility
+
+The aggregate operation is one of the most complex operations in the spec. Although implementations mostly agree on behaviors, there may be gaps in corner cases. Those behavioral differences are captured in compatibility.
+
+NOTE: The compatibility is meant to address gaps in the core implementation of aggregation such as grouping sets. For custom aggregations, consider using aggregate extension functions. If you want to introduce a new compatibility mode, reach out Substrait PMC to discuss.
+
+#### yield_no_rows_on_empty_input
+
+AggregateRel **MUST NOT** produce a row on empty input even if the `groupings` is empty or `groupings` include an empty grounping set.
+
+**Default:** ***false***. Both empty `groupings` or any empty grounping sets in `groupings` yield a row on empty input.
+**Compatibility for**: Microsoft SQL server family, Oracle.
+
+**Example:**
+```sql
+-- The following two SQL statements yields a single row with value 0 in the systems DO NOT require this compatibility.
+SELECT COUNT(*) FROM T -- [(0)] when T is empty.
+SELECT COUNT(*) FROM T GROUP BY GROUNPING SETS (()) -- [] when T is empty in systems requiring this compatibility.
+```
 
 === "AggregateRel Message"