Why TRY_PARSE its so slow? - sql-server

Question

I have this query that basically returns (right now) only 10 rows as results:
select *
FROM Table1 as o
inner join Table2 as t on t.Field1 = o.Field2
where Code = 123456 and t.FakeData is not null
Now, if I want to parse the field FakeData (which, unfortunately, can contain different types of data, from DateTime to Surname/etc; i.e. nvarchar(70)), for data show and/or filtering:
select *, TRY_PARSE(t.FakeData as date USING 'en-GB') as RealDate
FROM Table1 as o
inner join Table2 as t on t.Field1 = o.Field2
where Code = 123456 and t.FakeData is not null
It takes x10 the query to be executed.
Where am I wrong? How can I speed up?
I can't edit the database, I'm just a customer which read data.

Answer 1

The TSQL documentation for TRY_PARSE makes the following observation:
Keep in mind that there is a certain performance overhead in parsing the string value.
NB: I am assuming your typical date format would be dd/mm/yyyy.
The following is something of a shot-in-the-dark that might help. By progressively assessing the nvarchar column if it is a candidate as a date it is possible to reduce the number of uses of that function. Note that a data point established in one apply can then be referenced in a subsequent apply:
CREATE TABLE mytable(
FakeData NVARCHAR(60) NOT NULL
);
INSERT INTO mytable(FakeData) VALUES (N'oiwsuhd ouhw dcouhw oduch woidhc owihdc oiwhd cowihc');
INSERT INTO mytable(FakeData) VALUES (N'9603200-0297r2-0--824');
INSERT INTO mytable(FakeData) VALUES (N'12/03/1967');
INSERT INTO mytable(FakeData) VALUES (N'12/3/2012');
INSERT INTO mytable(FakeData) VALUES (N'3/3/1812');
INSERT INTO mytable(FakeData) VALUES (N'ohsw dciuh iuh pswiuh piwsuh cpiuwhs dcpiuhws ipdcu wsiu');
select
t.FakeData, oa3.RealDate
from mytable as t
outer apply (
select len(FakeData) as fd_len
) oa1
outer apply (
select case when oa1.fd_len > 10 then 0
when len(replace(FakeData,'/','')) + 2 = oa1.fd_len then 1
else 0
end as is_candidate
) oa2
outer apply (
select case when oa2.is_candidate = 1 then TRY_PARSE(t.FakeData as date USING 'en-GB') end as RealDate
) oa3
FakeData
RealDate
oiwsuhd ouhw dcouhw oduch woidhc owihdc oiwhd cowihc
null
9603200-0297r2-0--824
null
12/03/1967
1967-03-12
12/3/2012
2012-03-12
3/3/1812
1812-03-03
ohsw dciuh iuh pswiuh piwsuh cpiuwhs dcpiuhws ipdcu wsiu
null
db<>fiddle here