SQL Server - Instead of Insert Trigger - override truncation error? - sql

Question

Scenario: using an "Instead of Insert" trigger to a different table (overriding inserting into the current table) throws a truncation error.
Try to insert data into TableA
Instead Of Trigger is setup to run on TableA
This trigger instead inserts into TableB
TableA is not written to
Issue:
TableA has a nvarchar(10) desc column and TableB has a nvarchar(200) desc column. An insert trigger is setup on TableA where the data for the desc column is 50 characters long.
With SET ANSI_WARNINGS ON (the default), TableA causes a truncation error.
With SET ANSI_WARNINGS OFF (dangerous), truncation is ignored on TableB. So if the desc came in at 400 characters, it would be truncated to 200 characters with no errors.
Setup
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[results]
(
[id] [int] IDENTITY(1,1) NOT NULL,
[causefailure] [nvarchar](5) NULL,
[type] [nvarchar](50) NOT NULL,
[description] [nvarchar](200) NULL,
[rundate] [datetime] NOT NULL DEFAULT (getdate())
) ON [PRIMARY]
GO
CREATE TABLE [dbo].[test_table]
(
[id] [int] IDENTITY(1,1) NOT NULL,
[description] [nvarchar](10) NULL,
[rundate] [datetime] NOT NULL DEFAULT (getdate())
) ON [PRIMARY]
GO
CREATE TRIGGER [dbo].[InsteadTrigger]
ON [dbo].[test_table]
INSTEAD OF INSERT
AS
SET NOCOUNT ON
SET ANSI_WARNINGS ON
BEGIN
INSERT INTO results([type], [description])
SELECT
(CASE SUBSTRING([description], 1, 1)
WHEN 'a'
THEN 'causes failure or truncation'
ELSE ''
END AS [causefailure],
'Instead Of Trigger' AS [type],
[description]
FROM inserted
END;
GO
ALTER TABLE [dbo].[test_table] ENABLE TRIGGER [InsteadTrigger]
GO
First attempt:
SET ANSI_WARNINGS OFF
INSERT INTO [dbo].[test_table]([description])
SELECT 'atest12345678910' AS [description]
Second attempt:
SET ANSI_WARNINGS ON
INSERT INTO [dbo].[test_table]([description])
SELECT 'btest12345678910' AS [description]
INSERT INTO [dbo].[test_table]([description])
SELECT 'atest12345678910' AS [description]
But when I run the the first code snippet, causefailure gets truncated. But there is no error. Running the second snippet inserts one record in [results]. But it throws an exception on the second.
Is there a way to ignore: constraints, conversion issues, etc., exceptions with the initial write to [test_table]? But have exceptions for any work done in the trigger (e.g. inserting into [results] and maybe even the actual work to [test_tables])?
Edit: I do not want to change the column size of nvarchar(10) desc
on TableA. The ultimate goal might be if the "instead of trigger"
fails to insert on TableA, it saves to TableB. Or it might even change
the size of desc on failure dynamically inside the trigger and retry
the insert.
Hopefully helpful!

Answer 1

Below is an incomplete idea how to work-around the above problem. It kind of evasive. But interesting way test how far you can rework a database.
The below changes the schema around when a table is created or altered. On creating or altering a table, it will be swapped out for an indexed view after the table gets renamed.
ALTER TRIGGER trigger_CreateTable
ON DATABASE
AFTER CREATE_TABLE, ALTER_TABLE
AS
BEGIN
--SELECT EVENTDATA()
DECLARE #Prefix AS nvarchar(256) = N'PleaseUseView_'
DECLARE #Event AS XML = EVENTDATA()
DECLARE #SchemaName AS nvarchar(255) = (#Event.value('(/EVENT_INSTANCE/SchemaName)[1]', 'NVARCHAR(255)'))
DECLARE #TableName AS nvarchar(255) = (#Event.value('(/EVENT_INSTANCE/ObjectName)[1]', 'NVARCHAR(255)'))
DECLARE #ObjectType AS nvarchar(255) = (#Event.value('(/EVENT_INSTANCE/ObjectType)[1]', 'NVARCHAR(255)'))
DECLARE #TableWithSchema AS nvarchar(512) = '[' + #SchemaName + '].[' + #TableName + ']'
CREATE TABLE #SchemaBindingDependencies
(
[id] int NOT NULL IDENTITY,
[schema] nvarchar(256) NOT NULL,
[name] nvarchar(256) NOT NULL
)
INSERT INTO #SchemaBindingDependencies([schema], [name])
SELECT DISTINCT s.name AS [schema], o.name
FROM sys.objects AS o
INNER JOIN sysdepends AS d
ON d.id = o.[object_id]
INNER JOIN sys.schemas AS s
ON s.[schema_id] = o.[schema_id]
WHERE o.type ='V' AND d.depid = OBJECT_ID(#TableWithSchema)
AND SUBSTRING(#TableName, LEN(#Prefix) + 1, 256) LIKE o.[name]
IF (EXISTS(SELECT 1 FROM #SchemaBindingDependencies))
BEGIN
DECLARE #Index AS int = (SELECT MAX(id) FROM #SchemaBindingDependencies)
WHILE (#Index > 0)
BEGIN
DECLARE #ViewName1 AS nvarchar(256) = (SELECT [name] FROM #SchemaBindingDependencies WHERE id = #Index)
IF (#ViewName1 IS NOT NULL)
BEGIN
DECLARE #SchemaName1 AS nvarchar(256) = (SELECT [schema] FROM #SchemaBindingDependencies WHERE id = #Index)
DECLARE #DropSchemaBoundViewQuery AS nvarchar(1000) = 'DROP VIEW [' + #SchemaName + '].[' + #ViewName1 + ']'
EXEC(#DropSchemaBoundViewQuery)
END
SET #Index = #Index - 1
END
END
IF (SUBSTRING(#TableName, 1, LEN(#Prefix)) <> #Prefix)
BEGIN
DECLARE #NewTableName AS nvarchar(512) = #Prefix + #TableName + ''
DECLARE #NewTableWithSchema AS nvarchar(512) = '[' + #SchemaName + '].[' + #NewTableName + ']'
EXEC sp_rename #TableWithSchema, #NewTableName
SET #TableName = #NewTableName
SET #TableWithSchema = '[' + #SchemaName + '].[' + #NewTableName + ']'
END
DECLARE #Columns AS nvarchar(max) = (STUFF((SELECT ',[' + x.[name] + ']' FROM (
SELECT c.[name]
FROM sys.columns AS c
INNER JOIN sys.tables AS t
ON t.[object_id] = c.[object_id]
INNER JOIN sys.schemas AS s
ON s.[schema_id] = t.[schema_id]
WHERE t.[name] = #TableName AND s.[name] = #SchemaName) AS x FOR XML PATH('')), 1, 1, ''))
DECLARE #ViewName AS nvarchar(256) = SUBSTRING(#TableName, LEN(#Prefix) + 1, 256)
DECLARE #ViewWithSchema AS nvarchar(512) = '[' + #SchemaName + '].[' + #ViewName + ']'
DECLARE #Query AS nvarchar(max) =
N'CREATE VIEW ' + #ViewWithSchema + N' ' + CHAR(10) + CHAR(13) +
N'WITH SCHEMABINDING ' + CHAR(10) + CHAR(13) +
N'AS ' + CHAR(10) + CHAR(13) +
N' SELECT ' + #Columns + ' ' + CHAR(10) + CHAR(13) +
N' FROM ' + #TableWithSchema + N' '
--SELECT #Query
EXEC(#Query)
SET #Query =
N'CREATE UNIQUE CLUSTERED INDEX [CIX_' + #ViewName + N'] ' + CHAR(10) + CHAR(13) +
N'ON ' + #ViewWithSchema + N'(' + #Columns + N')'
EXEC(#Query)
-- TODO: Use the below double commented to build a variable insert statement for the "Instead of TRIGGER"
--
----DECLARE #tv_source TABLE (id int)
----declare #XML xml;
----set #XML =
---- (
---- select top(0) *
---- from #tv_source
---- for XML RAW, ELEMENTS, XMLSCHEMA
---- );
----SELECT T.c.query('.'), T.c.value('#name', 'nvarchar(256)')
----FROM #XML.nodes('/*/*/*/*/*') AS T(c)
--
--SET #Query =
-- N'CREATE TRIGGER [Trigger_' + #ViewName + N'] ' + CHAR(10) + CHAR(13) +
-- N'ON ' + #ViewWithSchema + N' ' + CHAR(10) + CHAR(13) +
-- N'INSTEAD OF INSERT ' + CHAR(10) + CHAR(13) +
-- N'AS BEGIN ' + CHAR(10) + CHAR(13) +
-- N'BEGIN TRY ' + CHAR(10) + CHAR(13) +
-- N' INSERT INTO ' + #TableWithSchema + N'(' + #Columns + N')'
-- N' SELECT ' + #Columns +
--EXEC(#Query)
END
Ideally, you would use a different schema for tables. And use dbo or
the default for the view.
Once the Instead of TRIGGER is working, you could wrap a TRY/CATCH around it. On the catch, check schema for truncation. And expand the column sizes if needed.
Incomplete solution. But it is the answer I will stick with for now.
If anyone has any better answers or a complete solution, please add it!
tl;dr
One interesting thing is the below query
DECLARE #tv_source TABLE (id int)
declare #XML xml;
select top(0) *
from #tv_source
for XML RAW, ELEMENTS, XMLSCHEMA
You can return schema. SOAP for the above XML. Or JsonSchema or Avro if using the SQL 2016+ Json version to build Restful API's with schema awareness. And with schema awareness, an application gateway could auto-grab many dispersed Micro-Rest API's into one seemly large Rest API.