Using indexes when comparing datetimes - sql-server

Question

I have two tables, both of which containing millions of rows of data.
tbl_one:
purchasedtm DATETIME,
userid INT,
totalcost INT
tbl_two:
id BIGINT,
eventdtm DATETIME,
anothercol INT
The first table has a clustered index on the first two columns: CLUSTERED INDEX tbl_one_idx ON(purchasedtm, userid)
The second one has a primary key on its ID column, and also a non-clustered index on the eventdtm column.
I want to run a query which looks for rows in which purchasedtm and eventdtm are on the same day.
Originally, I wrote my query as:
WHERE CAST(tbl_one.purchasedtm AS DATE) = CAST(tbl_two.eventdtm AS DATE)
But this was not going to use either of the two indexes.
Later, I changed my query to this:
WHERE tbl_one.purchasedtm >= CAST(tbl_two.eventdtm AS DATE)
AND tbl_one.purchasedtm < DATEADD(DAY, 1, CAST(tbl_two.eventdtm AS DATE))
This way, because only one side of the comparison is wrapped in a function, the other side can still use its index. Correct?
I also have some additional questions:
I can write the query the other way around too, i.e. keeping tbl_two.eventdtm untouched and wrapping tbl_one.purchasedtm in CAST(). Would that make a difference in performance?
If the answer to the previous question is yes is it because eventdtm has its own dedicated index, while looking up purcahsedtm would only be a partial index match?
Are there other factors I can take into consideration for deciding which of the two choices is better? (For example, if there are millions of rows in tbl_one but billions of rows in tbl_two, would that impact which column I should CAST and which one I should not?)
In genera, if you compare two columns that are both indexed, would we gain any performance compared to a similar scenario in which only one of them is indexed?
And lastly, can I perform my original task without using CAST?
Note: I do not have the ability to create or modify indexes, add columns, etc.

Answer 1

Little. late after commenting but...
As discussed in the comments, code such as CAST(DateTimeColumn AS date) is actually SARGable. Rob Farley posted an article on some of the SARGable and non-SARGable functionality here, however, I'll cover a few things off anyway.
Firstly, applying a function to a column will normally make your query non-SARGable, and especially if it changes the order of the values or the order of them is meaningless. Take something like:
SELECT *
FROM TABLE
WHERE RIGHT(COLUMN,5) = 'value';
The order of the values in the column are utterly unhelpful here, as we're focusing on the right hand characters. Unfortunately, as Rob also discusses:
SELECT *
FROM TABLE
WHERE LEFT(COLUMN,5) = 'value';
This is also non-SARGable. However what about the following?
SELECT *
FROM TABLE
WHERE Column LIKE 'value%';
This is, as the logic isn't applied to the column and the order doesn't change. If the value wehre '%value%' then that too would be non-SARGable.
When applying logic that adds (or subtracts) what you want to find, you always want to apply that to the literal value (or function, like GETDATE()`). For example one of these expressions is SARGable the other is not:
Column + 1 = #Variable --non-SARGable
Column = #Variable - 1 --SARGable
The same applies to things like DATEADD
#DateVariable BETWEEN DateColumn AND DATEADD(DAY, 30,DateColumn) --non-SARGable
DateColumn BETWEEN DATEADD(DAY, -30, #DateVariable) AND #DateVariable --SARGable
Changing the datatype (other than to a date) rarely will keep a query SARGable. CONVERT(date,varchardate,112) will not be SARGable, even though the order of the column is unchanged. Converting an decimal to an int, however, had the same result as converting a datetime to a date, and kept SARGability:
CREATE TABLE testtab (n decimal(2,1) PRIMARY KEY CLUSTERED);
INSERT INTO testtab
VALUES(0.1),
(0.3),
(1.1),
(1.7),
(2.4);
GO
SELECT n
FROM testtab
WHERE CONVERT(int,n) = 2;
GO
DROP TABLE testtab;
Hopefully, that gives you enough to go on, but pelase do ask if you want me to add anything further.