Add a warning to the CountRows function if a data source that caches its count is passed to it

src/libraries/Microsoft.PowerFx.Connectors/Tabular/ExternalCdpDataSource.cs

@@ -36,6 +36,8 @@ public ExternalCdpDataSource(DName name, string datasetName, ServiceCapabilities
 
         public TabularDataQueryOptions QueryOptions => new TabularDataQueryOptions(this);
 
+        public bool HasCachedCountRows => false;
+
         public string Name => EntityName.Value;
 
         public bool IsSelectable => ServiceCapabilities.IsSelectable;

src/libraries/Microsoft.PowerFx.Core/Entities/External/IExternalTabularDataSource.cs

@@ -11,6 +11,13 @@ internal interface IExternalTabularDataSource : IExternalDataSource, IDisplayMap
     {
         TabularDataQueryOptions QueryOptions { get; }
 
+        /// <summary>
+        /// Some data sources (like Dataverse) may return a cached value for
+        /// the number of rows (calls to CountRows) instead of always retrieving
+        /// the latest count.
+        /// </summary>
+        bool HasCachedCountRows { get; }
+
         IReadOnlyList<string> GetKeyColumns();
 
         IEnumerable<string> GetKeyColumns(IExpandInfo expandInfo);
@@ -23,4 +30,4 @@ internal interface IExternalTabularDataSource : IExternalDataSource, IDisplayMap
 
         bool CanIncludeExpand(IExpandInfo parentExpandInfo, IExpandInfo expandToAdd);
     }
-}
+}

src/libraries/Microsoft.PowerFx.Core/Localization/Strings.cs

@@ -745,6 +745,8 @@ internal static class TexlStrings
         public static ErrorResourceKey WarnDeferredType = new ErrorResourceKey("WarnDeferredType");
         public static ErrorResourceKey ErrColRenamedTwice_Name = new ErrorResourceKey("ErrColRenamedTwice_Name");
 
+        public static ErrorResourceKey WrnCountRowsMayReturnCachedValue = new ErrorResourceKey("WrnCountRowsMayReturnCachedValue");
+
         public static StringGetter InfoMessage = (b) => StringResources.Get("InfoMessage", b);
         public static StringGetter InfoNode_Node = (b) => StringResources.Get("InfoNode_Node", b);
         public static StringGetter InfoTok_Tok = (b) => StringResources.Get("InfoTok_Tok", b);

src/libraries/Microsoft.PowerFx.Core/Texl/Builtins/CountRows.cs

@@ -5,6 +5,7 @@
 using System.Linq;
 using Microsoft.PowerFx.Core.App.ErrorContainers;
 using Microsoft.PowerFx.Core.Binding;
+using Microsoft.PowerFx.Core.Binding.BindInfo;
 using Microsoft.PowerFx.Core.Entities;
 using Microsoft.PowerFx.Core.Errors;
 using Microsoft.PowerFx.Core.Functions;
@@ -68,6 +69,32 @@ public override bool IsServerDelegatable(CallNode callNode, TexlBinding binding)
             return TryGetValidDataSourceForDelegation(callNode, binding, out var dataSource, out var preferredFunctionDelegationCapability);
         }
 
+        public override void CheckSemantics(TexlBinding binding, TexlNode[] args, DType[] argTypes, IErrorContainer errors)
+        {
+            base.CheckSemantics(binding, args, argTypes, errors);
+            if (args[0] is not FirstNameNode node)
+            {
+                // No additional check
+                return;
+            }
+
+            var info = binding.GetInfo(args[0] as FirstNameNode);
+            if (info.Kind != BindKind.Data)
+            {
+                // No additional check
+                return;
+            }
+
+            if (argTypes[0].AssociatedDataSources?.Count == 1)
+            {
+                var associatedDataSource = argTypes[0].AssociatedDataSources.Single();
+                if (associatedDataSource.HasCachedCountRows)
+                {
+                    errors.EnsureError(DocumentErrorSeverity.Warning, node, TexlStrings.WrnCountRowsMayReturnCachedValue);
+                }
+            }
+        }
+
         // See if CountDistinct delegation is available. If true, we can make use of it on primary key as a workaround for CountRows delegation
         internal bool TryGetValidDataSourceForDelegation(CallNode callNode, TexlBinding binding, out IExternalDataSource dataSource, out DelegationCapability preferredFunctionDelegationCapability)
         {

src/strings/PowerFxResources.en-US.resx

@@ -4332,6 +4332,10 @@
     <value>Can't delegate {0}: contains a behavior function '{1}'.</value>
     <comment>Warning message.</comment>
   </data>
+  <data name="WrnCountRowsMayReturnCachedValue" xml:space="preserve">
+    <value>CountRows may return a cached value. Use CountIf(DataSource, true) to get the latest count.</value>
+    <comment>{Locked=CountRows}. Warning message when an expression with the CountRows function is used with a data source that caches its size.</comment>
+  </data>
   <data name="AboutIsMatch" xml:space="preserve">
     <value>Determines if the supplied text has a match of the supplied text format.</value>
     <comment>Description of 'IsMatch' function.</comment>

src/tests/Microsoft.PowerFx.Core.Tests.Shared/AssociatedDataSourcesTests/TestDVEntity.cs

@@ -15,6 +15,13 @@ namespace Microsoft.PowerFx.Core.Tests.AssociatedDataSourcesTests
 {
     public class AccountsEntity : IExternalEntity, IExternalDataSource
     {
+        private readonly bool _hasCachedCountRows;
+
+        public AccountsEntity(bool hasCachedCountRows = false)
+        {
+            this._hasCachedCountRows = hasCachedCountRows;
+        }
+
         public DName EntityName => new DName("Accounts");
 
         public string Name => "Accounts";
@@ -33,7 +40,7 @@ public class AccountsEntity : IExternalEntity, IExternalDataSource
 
         public bool IsClearable => true;
 
-        DType IExternalEntity.Type => AccountsTypeHelper.GetDType();
+        DType IExternalEntity.Type => AccountsTypeHelper.GetDType(this._hasCachedCountRows);
 
         IExternalDataEntityMetadataProvider IExternalDataSource.DataEntityMetadataProvider => throw new NotImplementedException();
 
@@ -54,14 +61,15 @@ internal static class AccountsTypeHelper
                                                          "name`Account Name`:s, numberofemployees:n, primarytwitterid:s, stockexchange:s, telephone1:s, telephone2:s, telephone3:s, tickersymbol:s, versionnumber:n, " +
                                                          "websiteurl:h, nonsearchablestringcol`Non-searchable string column`:s, nonsortablestringcolumn`Non-sortable string column`:s]";
 
-        public static DType GetDType()
+        public static DType GetDType(bool hasCachedCountRows = false)
         {
             DType accountsType = TestUtils.DT2(SimplifiedAccountsSchema);
             var dataSource = new TestDataSource(
                 "Accounts", 
                 accountsType, 
                 keyColumns: new[] { "accountid" },
-                selectableColumns: new[] { "name", "address1_city", "accountid", "address1_country", "address1_line1" });
+                selectableColumns: new[] { "name", "address1_city", "accountid", "address1_country", "address1_line1" },
+                hasCachedCountRows: hasCachedCountRows);
             var displayNameMapping = dataSource.DisplayNameMapping;            
             displayNameMapping.Add("name", "Account Name");
             displayNameMapping.Add("address1_city", "Address 1: City");

src/tests/Microsoft.PowerFx.Core.Tests.Shared/AssociatedDataSourcesTests/TestDelegationValidation.cs

@@ -6,6 +6,7 @@
 using System.Linq;
 using System.Text;
 using Microsoft.PowerFx.Core.Entities.QueryOptions;
+using Microsoft.PowerFx.Core.Errors;
 using Microsoft.PowerFx.Core.Tests.Helpers;
 using Microsoft.PowerFx.Core.Texl;
 using Microsoft.PowerFx.Types;
@@ -100,5 +101,38 @@ private void TestDelegableExpressions(Features features, string expression, bool
             // validate we can generate the display expression
             string displayExpr = engine.GetDisplayExpression(expression, symbolTable);
         }
+
+        [Theory]
+        [InlineData(true)]
+        [InlineData(false)]
+        public void TestCountRowsWarningForCachedData(bool isCachedData)
+        {
+            var symbolTable = new DelegatableSymbolTable();
+            symbolTable.AddEntity(new AccountsEntity(isCachedData));
+            var config = new PowerFxConfig(Features.PowerFxV1)
+            {
+                SymbolTable = symbolTable
+            };
+
+            var engine = new Engine(config);
+            var result = engine.Check("CountRows(Accounts)");
+            Assert.True(result.IsSuccess);
+
+            if (!isCachedData)
+            {
+                Assert.Empty(result.Errors);
+            }
+            else
+            {
+                Assert.Single(result.Errors);
+                var error = result.Errors.Single();
+                Assert.Equal(ErrorSeverity.Warning, error.Severity);
+            }
+
+            // Only shows warning if data source is passed directly to CountRows
+            result = engine.Check("CountRows(Filter(Accounts, IsBlank('Address 1: City')))");
+            Assert.True(result.IsSuccess);
+            Assert.Empty(result.Errors);
+        }
     }
 }

src/tests/Microsoft.PowerFx.Core.Tests.Shared/Helpers/TestTabularDataSource.cs

@@ -175,8 +175,9 @@ internal class TestDataSource : IExternalDataSource, IExternalTabularDataSource
         private readonly string[] _keyColumns;
         private readonly HashSet<string> _selectableColumns;
         private readonly TabularDataQueryOptions _tabularDataQueryOptions;
+        private readonly bool _hasCachedCountRows;
 
-        internal TestDataSource(string name, DType schema, string[] keyColumns = null, IEnumerable<string> selectableColumns = null)
+        internal TestDataSource(string name, DType schema, string[] keyColumns = null, IEnumerable<string> selectableColumns = null, bool hasCachedCountRows = false)
         {
             ExternalDataEntityMetadataProvider = new ExternalDataEntityMetadataProvider();
             Type = DType.AttachDataSourceInfo(schema, this);
@@ -185,10 +186,13 @@ internal TestDataSource(string name, DType schema, string[] keyColumns = null, I
             _keyColumns = keyColumns ?? Array.Empty<string>();
             _selectableColumns = new HashSet<string>(selectableColumns ?? Enumerable.Empty<string>());
             _tabularDataQueryOptions = new TabularDataQueryOptions(this);
+            _hasCachedCountRows = hasCachedCountRows;
         }
 
         public string Name { get; }
 
+        public bool HasCachedCountRows => this._hasCachedCountRows;
+
         public virtual bool IsSelectable => true;
 
         public virtual bool IsDelegatable => throw new NotImplementedException();