Just update one column, strange execution plan I have logged the slowest queries on my database, and one surprises me, showing many times in the list, and taking often many seconds to execute.

UPDATE books SET last_read='2018-12-31 11:23:45' WHERE book_id='15'


book_id is int identity PK (clustered), last_read is a datetime.
The query is written with the 15 in single quotes, thus requiring a conversion, but I cannot imagine this being a big deal, because the conversion would only be done once per query.
There are 6 indexes on the table, but the column last_read is not involved or included in any of them. The PK is on book_id and is nothing special.

The estimated execution plan tells me:

UPDATE: 0%
Clustered Index Update (on the PK constraint): 100%


I would not expect the query to update any index, because last_read is not indexed in any way, and book_id is not changed.

What am I missing?
Let's do a small thought exercise:


How do you think SQL Server finds the row with that book_id?
How do you think it finds (not updates) the value for last_read?


Remember that the clustered index is the table. If you update the table, you must update at least the clustered index.

Let me continue the thought exercise by asking a different question - do you think there is (or should be) a more efficient way of updating this value, than by finding the row via the clustered index, and changing the value there?

Let's leave heaps out of this for now, but the concept is similar there.