Stop wrapping single changes in transactions where possible

Closes dotnet#27439

src/EFCore.Relational/Properties/RelationalStrings.resx

@@ -669,6 +669,12 @@
  <data name="MissingParameterValue" xml:space="preserve">
  <value>No value was provided for the required parameter '{parameter}'.</value>
  </data>
+ <data name="ModificationCommandBatchAlreadyComplete" xml:space="preserve">
+ <value>Cannot add commands to a completed ModificationCommandBatch.</value>
+ </data>
+ <data name="ModificationCommandBatchNotComplete" xml:space="preserve">
+ <value>Cannot execute an ModificationCommandBatch which hasn't been completed.</value>
+ </data>
  <data name="ModificationCommandInvalidEntityState" xml:space="preserve">
  <value>Cannot save changes for an entity of type '{entityType}' in state '{entityState}'. This may indicate a bug in Entity Framework, please open an issue at https://go.microsoft.com/fwlink/?linkid=2142044. Consider using 'DbContextOptionsBuilder.EnableSensitiveDataLogging' to see the key values of the entity.</value>
  </data>

src/EFCore.Relational/Update/IUpdateSqlGenerator.cs

@@ -54,6 +54,26 @@ void AppendNextSequenceValueOperation(
  /// <param name="commandStringBuilder">The builder to which the SQL fragment should be appended.</param>
  void AppendBatchHeader(StringBuilder commandStringBuilder);
 
+ /// <summary>
+ /// Prepends a SQL command for turning on autocommit mode in the database, in case it is off.
+ /// </summary>
+ /// <param name="commandStringBuilder">The builder to which the SQL should be prepended.</param>
+ void PrependEnsureAutocommit(StringBuilder commandStringBuilder);
+
+ /// <summary>
+ /// Appends a SQL command for deleting a row to the commands being built.
+ /// </summary>
+ /// <param name="commandStringBuilder">The builder to which the SQL should be appended.</param>
+ /// <param name="command">The command that represents the delete operation.</param>
+ /// <param name="commandPosition">The ordinal of this command in the batch.</param>
+ /// <param name="requiresTransaction">Returns whether the SQL appended must be executed in a transaction to work correctly.</param>
+ /// <returns>The <see cref="ResultSetMapping" /> for the command.</returns>
+ ResultSetMapping AppendDeleteOperation(
+ StringBuilder commandStringBuilder,
+ IReadOnlyModificationCommand command,
+ int commandPosition,
+ out bool requiresTransaction);
+
  /// <summary>
  /// Appends a SQL command for deleting a row to the commands being built.
  /// </summary>
@@ -64,7 +84,22 @@ void AppendNextSequenceValueOperation(
  ResultSetMapping AppendDeleteOperation(
  StringBuilder commandStringBuilder,
  IReadOnlyModificationCommand command,
- int commandPosition);
+ int commandPosition)
+ => AppendDeleteOperation(commandStringBuilder, command, commandPosition, out _);
+
+ /// <summary>
+ /// Appends a SQL command for inserting a row to the commands being built.
+ /// </summary>
+ /// <param name="commandStringBuilder">The builder to which the SQL should be appended.</param>
+ /// <param name="command">The command that represents the delete operation.</param>
+ /// <param name="commandPosition">The ordinal of this command in the batch.</param>
+ /// <param name="requiresTransaction">Returns whether the SQL appended must be executed in a transaction to work correctly.</param>
+ /// <returns>The <see cref="ResultSetMapping" /> for the command.</returns>
+ ResultSetMapping AppendInsertOperation(
+ StringBuilder commandStringBuilder,
+ IReadOnlyModificationCommand command,
+ int commandPosition,
+ out bool requiresTransaction);
 
  /// <summary>
  /// Appends a SQL command for inserting a row to the commands being built.
@@ -76,7 +111,22 @@ ResultSetMapping AppendDeleteOperation(
  ResultSetMapping AppendInsertOperation(
  StringBuilder commandStringBuilder,
  IReadOnlyModificationCommand command,
- int commandPosition);
+ int commandPosition)
+ => AppendInsertOperation(commandStringBuilder, command, commandPosition, out _);
+
+ /// <summary>
+ /// Appends a SQL command for updating a row to the commands being built.
+ /// </summary>
+ /// <param name="commandStringBuilder">The builder to which the SQL should be appended.</param>
+ /// <param name="command">The command that represents the delete operation.</param>
+ /// <param name="commandPosition">The ordinal of this command in the batch.</param>
+ /// <param name="requiresTransaction">Returns whether the SQL appended must be executed in a transaction to work correctly.</param>
+ /// <returns>The <see cref="ResultSetMapping" /> for the command.</returns>
+ ResultSetMapping AppendUpdateOperation(
+ StringBuilder commandStringBuilder,
+ IReadOnlyModificationCommand command,
+ int commandPosition,
+ out bool requiresTransaction);
 
  /// <summary>
  /// Appends a SQL command for updating a row to the commands being built.
@@ -88,5 +138,6 @@ ResultSetMapping AppendInsertOperation(
  ResultSetMapping AppendUpdateOperation(
  StringBuilder commandStringBuilder,
  IReadOnlyModificationCommand command,
- int commandPosition);
+ int commandPosition)
+ => AppendUpdateOperation(commandStringBuilder, command, commandPosition, out _);
 }

src/EFCore.Relational/Update/Internal/BatchExecutor.cs

@@ -52,6 +52,16 @@ public virtual int Execute(
  IEnumerable<ModificationCommandBatch> commandBatches,
  IRelationalConnection connection)
  {
+ using var batchEnumerator = commandBatches.GetEnumerator();
+
+ if (!batchEnumerator.MoveNext())
+ {
+ return 0;
+ }
+
+ var currentBatch = batchEnumerator.Current;
+ var nextBatch = batchEnumerator.MoveNext() ? batchEnumerator.Current : null;
+
  var rowsAffected = 0;
  var transaction = connection.CurrentTransaction;
  var beganTransaction = false;
@@ -62,7 +72,9 @@ public virtual int Execute(
  if (transaction == null
  && transactionEnlistManager?.EnlistedTransaction is null
  && transactionEnlistManager?.CurrentAmbientTransaction is null
- && CurrentContext.Context.Database.AutoTransactionsEnabled)
+ && CurrentContext.Context.Database.AutoTransactionsEnabled
+ // Don't start a transaction if we have a single batch which doesn't require a transaction (single command), for perf.
+ && (nextBatch is not null || currentBatch.RequiresTransaction))
  {
  transaction = connection.BeginTransaction();
  beganTransaction = true;
@@ -79,10 +91,13 @@ public virtual int Execute(
  }
  }
 
- foreach (var batch in commandBatches)
+ while (currentBatch is not null)
  {
- batch.Execute(connection);
- rowsAffected += batch.ModificationCommands.Count;
+ currentBatch.Execute(connection);
+ rowsAffected += currentBatch.ModificationCommands.Count;
+
+ currentBatch = nextBatch;
+ nextBatch = batchEnumerator.MoveNext() ? batchEnumerator.Current : null;
  }
 
  if (beganTransaction)
@@ -147,6 +162,16 @@ public virtual async Task<int> ExecuteAsync(
  IRelationalConnection connection,
  CancellationToken cancellationToken = default)
  {
+ using var batchEnumerator = commandBatches.GetEnumerator();
+
+ if (!batchEnumerator.MoveNext())
+ {
+ return 0;
+ }
+
+ var currentBatch = batchEnumerator.Current;
+ var nextBatch = batchEnumerator.MoveNext() ? batchEnumerator.Current : null;
+
  var rowsAffected = 0;
  var transaction = connection.CurrentTransaction;
  var beganTransaction = false;
@@ -157,7 +182,9 @@ public virtual async Task<int> ExecuteAsync(
  if (transaction == null
  && transactionEnlistManager?.EnlistedTransaction is null
  && transactionEnlistManager?.CurrentAmbientTransaction is null
- && CurrentContext.Context.Database.AutoTransactionsEnabled)
+ && CurrentContext.Context.Database.AutoTransactionsEnabled
+ // Don't start a transaction if we have a single batch which doesn't require a transaction (single command), for perf.
+ && (nextBatch is not null || currentBatch.RequiresTransaction))
  {
  transaction = await connection.BeginTransactionAsync(cancellationToken).ConfigureAwait(false);
  beganTransaction = true;
@@ -174,10 +201,13 @@ public virtual async Task<int> ExecuteAsync(
  }
  }
 
- foreach (var batch in commandBatches)
+ while (currentBatch is not null)
  {
- await batch.ExecuteAsync(connection, cancellationToken).ConfigureAwait(false);
- rowsAffected += batch.ModificationCommands.Count;
+ await currentBatch.ExecuteAsync(connection, cancellationToken).ConfigureAwait(false);
+ rowsAffected += currentBatch.ModificationCommands.Count;
+
+ currentBatch = nextBatch;
+ nextBatch = batchEnumerator.MoveNext() ? batchEnumerator.Current : null;
  }
 
  if (beganTransaction)

src/EFCore.Relational/Update/Internal/CommandBatchPreparer.cs

@@ -83,6 +83,8 @@ public virtual IEnumerable<ModificationCommandBatch> BatchCommands(
  batch.ModificationCommands.SelectMany(c => c.Entries), batch.ModificationCommands.Count);
  }
 
+ batch.Complete();
+
  yield return batch;
  }
  else
@@ -92,7 +94,10 @@ public virtual IEnumerable<ModificationCommandBatch> BatchCommands(
 
  foreach (var command in batch.ModificationCommands)
  {
- yield return StartNewBatch(parameterNameGenerator, command);
+ batch = StartNewBatch(parameterNameGenerator, command);
+ batch.Complete();
+
+ yield return batch;
  }
  }
 
@@ -109,6 +114,8 @@ public virtual IEnumerable<ModificationCommandBatch> BatchCommands(
  batch.ModificationCommands.SelectMany(c => c.Entries), batch.ModificationCommands.Count);
  }
 
+ batch.Complete();
+
  yield return batch;
  }
  else
@@ -118,7 +125,10 @@ public virtual IEnumerable<ModificationCommandBatch> BatchCommands(
 
  foreach (var command in batch.ModificationCommands)
  {
- yield return StartNewBatch(parameterNameGenerator, command);
+ batch = StartNewBatch(parameterNameGenerator, command);
+ batch.Complete();
+
+ yield return batch;
  }
  }
  }

src/EFCore.Relational/Update/ModificationCommandBatch.cs

@@ -33,6 +33,16 @@ public abstract class ModificationCommandBatch
  /// </returns>
  public abstract bool AddCommand(IReadOnlyModificationCommand modificationCommand);
 
+ /// <summary>
+ /// Indicates that no more commands will be added to this batch, and prepares it for execution.
+ /// </summary>
+ public abstract void Complete();
+
+ /// <summary>
+ /// Indicates whether the batch requires a transaction in order to execute correctly.
+ /// </summary>
+ public abstract bool RequiresTransaction { get; }
+
  /// <summary>
  /// Sends insert/update/delete commands to the database.
  /// </summary>