SQL Query earliest date in a table

Question

Hi
I have a table that has over 500,000,000 rows listed in it. The database was not designed by myself but supplied by a third party company for one of there quality products.
Within this table there are various fields, one of these fields is a date field of when the record was created.
Currently trying to run a deletion job on the table to delete a number of the old rows.
What I am looking for is the best way to get the earliest date from all of the rows, and return this is to a variable, so that I can use the dateadd function to delete any records before the last 30 days.
Due to the size of the table, any query I run takes a while, so my question is what would be the fastest way to get the earliest date?
Regards

Answer 1

Out of 500 million rows, what will be your total number of rows to be deleted?
Calculate as below:
SELECT COUNT(1)
FROM YourTable (NOLOCK)
WHERE dt <= DATEADD(d,-30,@dt);
If this number is huge (Lets say 250 million +) then it is better to:
1. Insert last 30 days rows in a temp table t1
SELECT * into dbo.t1
FROM YourTable (NOLOCK)
WHERE dt > DATEADD(d,-30,@dt);
2. Truncate YourTable
TRUNCATE TABLE YourTable
3. Load last 30 days data from temp table to YourTable:
INSERT INTO YourTable
SELECT *
FROM dbo.t1
4. Validate YourTable and then Drop the temp table:
DROP TABLE dbo.t1
You can go this route, when YourTable is idle (I mean, there should not be any data load going on this table).
If you still wish to go with delete apporach then create a sql agent job with
Uri Dimant's query and execute it on the server.
All the Best!
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