SQL 2008 R2 Unstable query

Question

Hello everyone,
I have a query that selects no data but if fails when there's an order by clause or if there's a conversion of hardcoded date.
The error looks like this: 
This only happens when the table is indexed.
I put a repro code below.
Could you please let me know if the error reprodces itself in your environment and your thoughts why ?
Thanks in advace.
The code below reproduces the issue:
CREATE TABLE [dbo].[Test](
[ID] [varchar](20) NOT NULL,
[DateStr] [varchar](20) NOT NULL,
[status] [varchar](6) NOT NULL,
) ON [PRIMARY]
GO
CREATE NONCLUSTERED INDEX [IX_Test] ON [dbo].Test
[ID] ASC
--, [coverage] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
GO
Insert Test
ID
, DateStr
, status
Values
'policy number'
, 'paid date'
, 'status'
-- This query fails
select *
, datepart(mm,Convert(datetime, '2015-03-24 23:59:00.000', 120))
from Test
where status = 'I'
and convert(datetime,DateStr,103) < Convert(datetime, '2015-03-24 23:59:00.000', 120) --@curr_date--'2015-03-24 23:59:00.000'
order by ID
-- This query works
select *
--, datepart(mm,Convert(datetime, '2015-03-24 23:59:00.000', 120))
from Test
where status = 'I'
and convert(datetime,DateStr,103) < Convert(datetime, '2015-03-24 23:59:00.000', 120) --@curr_date--'2015-03-24 23:59:00.000'
order by ID
-- This query also works
select *
, datepart(mm,Convert(datetime, '2015-03-24 23:59:00.000', 120))
from Test
where status = 'I'
and convert(datetime,DateStr,103) < Convert(datetime, '2015-03-24 23:59:00.000', 120) --@curr_date--'2015-03-24 23:59:00.000'
--order by ID
-- If I drop the index all queries work
Drop index Test.IX_Test
Conversion failed when converting date and/or time from character string.
SSIS question

Answer 1

There is no guaranteed order of evaluation, so a query with a potential conversion error in it may succeed or fail depending on the query plan, phase of the moon etc. This also applies if the conversion is in the SELECT list, and the WHERE clause filters
out the bad data. I'm not sure that I like the latter part, but that is just the way it works.
The only way to filter out bad data in SQL 2008 is by using CASE:
CASE WHEN isdate(col) = 1 THEN convert(datetime, col) ELSE NULL END
This is needed both in the WHERE clause and in the SELECT list.
It can be problematic if isdate returns 0, but the string converts according to the selected format code or vice versa.
On SQL 2012 and later, because here you can use try_convert which returns NULL if the string does not convert.
A general piece of advice is that you should not store dates or numbers in varchar columns. It is bound to cause misery and grief.
Erland Sommarskog, SQL Server MVP, [email protected]