Is There Is Any Performance Issue While Using Isnull() in SQL Server

Is there is any Performance issue while using ISNULL() in SQL Server?

If you need to use it, then any differences between ISNULL and alternatives like COALESCE or CASE are minuscule. Don't worry about it

Any differences come from how datatypes are handled. COALESCE/CASE can add implicit data type conversions whereas ISNULL has simpler rules.

Edit

ISNULL in the SELECT list to suppress NULLS is trivial. The main work will done in processing rows and data. An extra ISNULL won't be measurable: Don't optimise prematurely

Performance issue using IsNull function in the Select statement

I have no idea what your query is supposed to be doing. But this process:

ActualEvaluationPrice is the price for instrumentX at dateY. If this value is missing from HistoricPrice table then I find the previous price for instrumentX.

is handled easily with lag():

select vhiv.*
       coalesce(vhiv.ActualEvaluationPrice,
                lag(vhiv.ActualEvaluationPrice) over (partition by vhiv.InstrumentId order by DateValue)
               ) as UsedEvaluationPrice
from ViewHistoricInstrumentValue vhiv;

Note: If you need to filter out certain dates by joining to ValidDates, you can include the JOIN in the query. However, that is not part of the problem statement.

Performance of ISNULL() in GROUP BY clause SQL

Non-aggregated columns in SELECT clauses generally must precisely match the ones in GROUP BY clauses. If I were you, and I were dealing with tested production code, I would not make the change you propose.

Edit the match between non-aggregated SELECT columns and GROUP BY columns is necessary for GROUP BY. If the columns in SELECT are 1:1 dependent on the columns in GROUP BY, it will work. Otherwise the results are ambiguous.

ISNULL slows down query

When you include a field inside of a function, it changes how the optimizer has to run and forces it to ignore indexes.

see here: What makes a SQL statement sargable?

use of Isnull() in optimization

You can obtain an execution plan for your query to determine the cost of that operation.

How to do it and the output format really depends on each database product.

In the case of SQL server it may look like:

My intuition tells me that operation in particular should not be expensive. What would rather be expensive is to query a column that is not indexed.

Can using isnull in a where statement cause problems with using indexes?

Yes, any function calls in your WHERE clause will likely make the index useless. Try to rewrite it so that the index can be used:

SELECT t1.v3, t2.v2
FROM t1
INNER JOIN t2
ON t1.v1 = t2.v1
WHERE NOT t1.DeleteFlag = 'Y'

The index makes sense if the number of results you expect from the query is much smaller than the total number of rows in the table.

Why IsNull is twice slow as coalesce (same query)?

I wonder if you'd see an improvement by splitting the cases out explicitly:

...
AND ((t1.vchCol1 = t2.vchCol1) OR (t1.vchCol1 IS NULL AND t2.vchCol1 IS NULL))
AND ((t1.vchCol2 = t2.vchCol2) OR (t1.vchCol2 IS NULL AND t2.vchCol2 IS NULL))
...

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