Fix PostgreSql does not like 2 deletes in a single command.

src/Akka.Persistence.PostgreSql/Journal/PostgreSqlQueryExecutor.cs

@@ -19,6 +19,8 @@
 using System.Linq;
 using System.Runtime.CompilerServices;
 using System.Text;
+using System.Threading;
+using System.Threading.Tasks;
 
 namespace Akka.Persistence.PostgreSql.Journal
 {
@@ -61,6 +63,16 @@ SELECT MAX(e.{Configuration.SequenceNrColumnName}) as SeqNr FROM {Configuration.
                     UNION
                     SELECT MAX(m.{Configuration.SequenceNrColumnName}) as SeqNr FROM {Configuration.FullMetaTableName} m WHERE m.{Configuration.PersistenceIdColumnName} = @PersistenceId) as u";
 
+            // As per https://github.com/akkadotnet/Akka.Persistence.PostgreSql/pull/72, apparently PostgreSQL does not like
+            // it when you chain two deletes in a single command, so we have to split it into two.
+            // The performance penalty should be minimal, depending on the network speed
+            DeleteBatchSql = $@"
+                DELETE FROM {Configuration.FullJournalTableName}
+                WHERE {Configuration.PersistenceIdColumnName} = @PersistenceId AND {Configuration.SequenceNrColumnName} <= @ToSequenceNr;";
+
+            DeleteBatchSqlMetadata = $@"DELETE FROM {Configuration.FullMetaTableName}
+                WHERE {Configuration.PersistenceIdColumnName} = @PersistenceId AND {Configuration.SequenceNrColumnName} <= @ToSequenceNr;";
+
             switch (configuration.StoredAs)
             {
                 case StoredAsType.ByteA:
@@ -108,6 +120,8 @@ SELECT MAX(e.{Configuration.SequenceNrColumnName}) as SeqNr FROM {Configuration.
         protected override string CreateEventsJournalSql { get; }
         protected override string CreateMetaTableSql { get; }
         protected override string HighestSequenceNrSql { get; }
+        protected override string DeleteBatchSql { get; }
+        protected virtual string DeleteBatchSqlMetadata { get; }
 
         protected override void WriteEvent(DbCommand command, IPersistentRepresentation e, IImmutableSet<string> tags)
         {
@@ -190,6 +204,50 @@ protected override IPersistentRepresentation ReadEvent(DbDataReader reader)
 
             return new Persistent(deserialized, sequenceNr, persistenceId, manifest, isDeleted, ActorRefs.NoSender, null, timestamp);
         }
+
+        public override async Task DeleteBatchAsync(DbConnection connection, CancellationToken cancellationToken, string persistenceId, long toSequenceNr)
+        {
+            using (var deleteCommand = GetCommand(connection, DeleteBatchSql))
+            using (var deleteMetadataCommand = GetCommand(connection, DeleteBatchSqlMetadata))
+            using (var highestSeqNrCommand = GetCommand(connection, HighestSequenceNrSql))
+            {
+                AddParameter(highestSeqNrCommand, "@PersistenceId", DbType.String, persistenceId);
+
+                AddParameter(deleteCommand, "@PersistenceId", DbType.String, persistenceId);
+                AddParameter(deleteCommand, "@ToSequenceNr", DbType.Int64, toSequenceNr);
+
+                AddParameter(deleteMetadataCommand, "@PersistenceId", DbType.String, persistenceId);
+                AddParameter(deleteMetadataCommand, "@ToSequenceNr", DbType.Int64, toSequenceNr);
+
+                using (var tx = connection.BeginTransaction())
+                {
+                    deleteCommand.Transaction = tx;
+                    deleteMetadataCommand.Transaction = tx;
+                    highestSeqNrCommand.Transaction = tx;
+
+                    var res = await highestSeqNrCommand.ExecuteScalarAsync(cancellationToken);
+                    var highestSeqNr = res is long ? Convert.ToInt64(res) : 0L;
+
+                    await deleteCommand.ExecuteNonQueryAsync(cancellationToken);
+                    await deleteMetadataCommand.ExecuteNonQueryAsync(cancellationToken);
+
+                    if (highestSeqNr <= toSequenceNr)
+                    {
+                        using (var updateCommand = GetCommand(connection, UpdateSequenceNrSql))
+                        {
+                            updateCommand.Transaction = tx;
+
+                            AddParameter(updateCommand, "@PersistenceId", DbType.String, persistenceId);
+                            AddParameter(updateCommand, "@SequenceNr", DbType.Int64, highestSeqNr);
+
+                            await updateCommand.ExecuteNonQueryAsync(cancellationToken);
+                            tx.Commit();
+                        }
+                    }
+                    else tx.Commit();
+                }
+            }
+        }
     }
 
     public class PostgreSqlQueryConfiguration : QueryConfiguration