When the engine tries to insert a new row into the indexed table, being the index UNIQUE or not, it performs the following steps: The check mentioned on step 2 is free: traversing an index implies comparing the index keys anyway. This operation can be parallelized, and it is in fact: note that CPU time is twise the elapsed time. Arabic / عربية Kazakh / Қазақша If this is the case, how a unique non-clustered index would be beneficial over a non-unique non-clusterd index. Just like you. Reference: Pinal Dave (https://blog.sqlauthority.com). A UNIQUE index is built on uval and a plain index is built on ival. I thought that ‘Scan’ counted the number of times the object was scanned. The simple answer is that SQL Server will make sure that only one row exists for a given value if you declare the index as UNIQUE, otherwise you can have as many rows as you want with the same value. Thanks for pointing it out. Primary key vs. unique index: performance difference with foreign keys? Regarding your statement "a PRIMARY KEY constraint will either create an. If you’re not using the column to do anything (join, filter, order), then perhaps not. Not only does the query for the unique indexes choose a much nicer merge join, it doesn’t even get considered for parallelilzazation going parallel. Fragmentation information on these indexes shows the following: As we can see, these indexes are essentially the same. mmhhh... to enforce uniqueness perhaps? However, as mentioned earlier in this blog post, I was not allowed to change the code and that lead me to look at their table structure a bit more in-depth. You have more of them now, if any of this was enlightening to you. the uniqueness of data values, a unique index can also be used to improve data retrieval performance during query processing. Unique Index or Primary Key Index are physical structure that maintain uniqueness over some combination of columns across all rows of a table. This is the internet. Or if video is more your thing, check out Connor's latest video and Chris's latest video from their Youtube channels. English / English Portuguese/Portugal / Português/Portugal Expertise through exercise! To me, while I was checking with DBCC IND and PAGE, I could see the above situation. Isn't the RID and the "uniquifier" pretty much equivalent from an overhead standpoint, being a 4 byte identifier? I’m just going with simple joins here, since they seem like a pretty approachable subject to most people who are writing queries and creating indexes. Can too many unique indexes on a table, trailing with the primary key column for uniqueness, have a negative performance impact? There’s some code below, along with some screen shots, but…. As we had gone through the server configurations and database settings along with the indexes the option was to look at the query. Hi Erik, What about the known impact of UNIQUE index on CDC operation codes? So a non-unique index that has no duplicate values will look exactly the same as a unique index. I think it depends a bit on the data you’re indexing, and how cool your server is. When you look into the DMV, you can see that the unique non-clustered index has a record length of 107 bytes and the non-unique non-clustered index has a record length of 111 bytes. You said “ reducing the IO from the disk by few hundreds of the pages.”. I am sure the scenario exists somewhere and this is where I will request you to share the details with me and I will publish on this blog with due credit to you. Okei, thank for your post. Same deal with those. To start with, the RID's not 4 bytes. The nice thing is that a little uniqueness goes a long way. Ask Question Asked 8 years, 2 months ago. Connor and Chris don't just spend all day on AskTOM. pinal @ SQLAuthority.com, Table ‘Table1’. The uniquifier is a 4-byte sequential value that only appears on the rows that have duplicate clustered index key values (the first row SQL encounters won't have a uniquifier, any rows subsequent with the same clustering key will gain one, sequential value, starting at (I believe) 1. Good Day, Sir! Catalan / Català If the optimiser knows the index is UNIQUE, how will that affect the query plan?