Clustered index scans should definitely be adversely affected by masses of empty space in pages from deleted rows, since the scan will have to read lots more pages than...

Could you try

WHERE [AccountNumber] = CONVERT(varchar(10), @AN)

?

When you plan to rebuild/reorganise an index you need to answer the main question:

What happens next?

Massive DELETEs on the table imply that there are as massive INSERTs.

What happens to the...

LibreOffice is not expensive at all

How massive are your DELETEs?

Probably it would be easier to copy the remaining part of data to a new table (exact copy of the old one) and then drop old...

It's actually another way around.

Bigger code structured into many nested stored procedures containing smaller chunks of the code will almost certainly give better performance.

Beginning with the compilation stage: long code...

Did you try to pass the parameter not via a literal constant but by a VARCHAR variable?

When you search by a value in non-indexed column what can you see in the query plan?

It's "Table scan" for HEAPs, and "Clustered Index Scan" for clustered tables.

does it make...

Data pages make up a clustering key. No data page exists outside a clustering key.

if any data in a table with a clustering key is updated then some part of...

Then it would be more efficient to copy the data to remain in the table to a newly created table of the same structure, delete the old table and rename...

How significant is the part of the data you want to delete?

5%, half, 90%?

3) Make sure you  update only values which have actually changed.

When choosing a plan optimiser has to estimate the data sets coming out of each of the objects (tables, views, functions) joined in the query.

it uses indexes, statistics, to get...

That's how it does not work.

It would work if the reality would match the assumptions used for the model.

If the reality does not match the assumptions then the model does...

These are the tasks we need to do:

For each user, determine their country based on the IP address

Fail on the first step. No point of going any further.

Almost every browser...