PostgreSQL 错误:EXECUTE 的查询字符串参数为空

我有一个名为 evidence 的表，它带有一个触发器，它调用一个存储过程，该过程基本上按月进行表分区。但是，当我开始在负载下插入大量行时，我遇到了一个模糊的错误：

Npgsql.NpgsqlException: query string argument of EXECUTE is null 
Severity: ERROR Code: 22004 at Npgsql.NpgsqlState.<ProcessBackendResponses_Ver_3>d__a.MoveNext() in c:'C#Apps'github.npgsql.Npgsql.stock'src'Npgsql'NpgsqlState.cs:line890 at Npgsql.ForwardsOnlyDataReader.GetNextResponseObject() in c:'C#Apps'github.npgsql.Npgsql.stock'src'Npgsql'NpgsqlDataReader.cs:line 1175 at 
    Npgsql.ForwardsOnlyDataReader.GetNextRowDescription() in c:'C#Apps'github.npgsql.Npgsql.stock'src'Npgsql'NpgsqlDataReader.cs:line 1191    at 
    Npgsql.ForwardsOnlyDataReader.NextResult() in c:'C#Apps'github.npgsql.Npgsql.stock'src'Npgsql'NpgsqlDataReader.cs:line 1377    at 
    Npgsql.NpgsqlCommand.ExecuteNonQuery() in c:'C#Apps'github.npgsql.Npgsql.stock'src'Npgsql'NpgsqlCommand.cs:line523

我的系统具有自动重试功能，最终每条记录都会插入到数据库中，但是在负载很高时会出现许多异常。

数据库是 CentOS 6 服务器上的 PostgreSQL 9.3，客户端是使用 Npgsql 驱动程序的 C# .NET。

桌子：

CREATE TABLE evidence
(
  id uuid NOT NULL,
  notification_id uuid NOT NULL,
  feedback character varying(200),
  result character varying(20),
  trigger_action_type character varying(200),
  trigger_action_id uuid,
  data_type integer NOT NULL,
  data bytea,
  name character varying(30),
  CONSTRAINT pk_evidence PRIMARY KEY (id)
);

触发：

CREATE TRIGGER evidence_move_to_partition_tables
  BEFORE INSERT
  ON evidence
  FOR EACH ROW
  EXECUTE PROCEDURE partition_evidence_by_month();

触发功能：

CREATE OR REPLACE FUNCTION partition_evidence_by_month()
  RETURNS trigger AS
$BODY$
DECLARE 
    _notification_id uuid;
    _raised_local_time timestamp without time zone;
    _table_name character varying(35);
    _start_date timestamp without time zone;
    _end_date timestamp without time zone;
    _table_space character varying(50) := 'ls_tablespace2';
    _query text;
BEGIN
    _notification_id := NEW.notification_id;
    SELECT raised_local_time FROM notifications WHERE id=_notification_id INTO _raised_local_time;
    _start_date := date_trunc('month', _raised_local_time);
    _end_date   := _start_date + '1 month'::interval;
    _table_name := 'evidence-' || to_char(_start_date, 'YYYY-MM');
    -- check to see if table already exists
    PERFORM 1
    FROM   pg_catalog.pg_class c
    JOIN   pg_catalog.pg_namespace n ON n.oid = c.relnamespace
    WHERE  c.relkind = 'r'
    AND    c.relname = _table_name
    AND    n.nspname = 'public';
    -- if the table doesn't exist, then create it now
    IF NOT FOUND THEN
        -- create partition table
        _query := 'CREATE TABLE public.' || quote_ident(_table_name) || ' ( ) INHERITS (public.evidence)';
        EXECUTE _query;
        -- alter owner
        --EXECUTE 'ALTER TABLE public.' || quote_ident(_table_name) || ' OWNER TO postgres';
        -- add index
        --EXECUTE 'ALTER TABLE public.' || quote_ident(_table_name) || ' ADD PRIMARY KEY (id)';
    END IF;
    -- move the data to the partition table
    EXECUTE 'INSERT INTO public.' || quote_ident(_table_name) || ' VALUES ($1.*)' USING NEW;
    RETURN NULL;
END;
$BODY$ LANGUAGE plpgsql VOLATILE COST 100;

区号：

using (var cmd = db.CreateCommand())
{
    cmd.CommandText = @"INSERT INTO evidence   
    (id, notification_id, feedback, result, trigger_action_type, 
     trigger_action_id, data_type, data, name) 
    VALUES (@id,@nid,@feedback,@result,@tat,@taid,@dt,@data,@name)";
    cmd.Parameters.AddWithValue("@id", evItem.ID);
    cmd.Parameters.AddWithValue("@nid", evItem.NotificationID);
    cmd.Parameters.AddWithValue("@feedback", evItem.Feedback);
    cmd.Parameters.AddWithValue("@result", evItem.Result);
    cmd.Parameters.AddWithValue("@tat", evItem.TriggerActionType);
    cmd.Parameters.AddWithValue("@taid", evItem.TriggerActionID);
    cmd.Parameters.AddWithValue("@dt", (int)evItem.DataType);
    cmd.Parameters.AddWithValue("@data", evItem.Data);
    cmd.Parameters.AddWithValue("@name", evItem.Name);
    cmd.ExecuteNonQuery();
}

为什么只有在系统负载不足时才会出现这种奇怪的错误？我该怎么做才能防止它发生？

谢谢！