unique vs non unique index performance
You can see in the statistics details the logical reads are much lesser in table 2 where we do Unique non-clustered Indexes. Why? ----------------------------------------------------The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood... Theodore Roosevelt, The Scary DBAAuthor of: SQL Server 2017 Query Performance Tuning, 5th Edition and SQL Server Execution Plans, 3rd EditionProduct Evangelist for Red Gate Software. If we look at the non clustered index structure, BTree - RID + TAID + Date + Uniquifier (20+8+8+4) total of = 40 Bytes, Leaf - RID + TAID + Date + Uniquifier (20+8+8+4) total of = 40 Bytes. That information, along with your comments, will be governed by Part 2: Training Budgets. Even one unique index can make a lot of difference, when joined with a non-unique index. I live in California with my wife Erika. This operation can be parallelized, and it is in fact: note that CPU time is twise the elapsed time. Czech / Čeština Is this only a factor if the data in the index is “naturally” unique; or will it also affect indexes if using the the ‘make my index unique’ unique flag when creating indexes? Twitter: @AnyWayDBA. Korean / 한국어 Nope. Your email address will not be published. Never stop learning, even if it hurts. Winner: Unique Indexes by saving overall resources. Wait, no, that’s something else. At leaf level, non-clustered key + clustered key +Uniquifier. No. The query plan uses INDEX SCAN on uval with a CLUSTERED INDEX SEEK to retrieve other values and TOP N to limit the number of rows returned. At leaf level, non-clustered key + clustered key, If I have a unique clustered index index and unique non-clusterd index indexthen my root level will have non-clustered key . Portuguese/Portugal / Português/Portugal Obviously there are no clustered indexes involved here. I will leave consistency issues aside, focusing only on performance details. Q: What was the last thing the Medic said to the Heavy? Some concern about PK and Index correct me if i am wrong when we create table index is created implicitly whether we set a PK or not...if its the case y we need to create Index explicitly if i think that explicit index is good for performance than what do we use for imlicit one...Thanks alot. Answered by: Tom Kyte - Last updated: September 05, 2017 - 11:50 am UTC, Reviewer: Alberto Dell'Era from Milan, Italy, Reviewer: Alberto Dell'Era from Milano, Italy, Reviewer: David Rydzewski from Lexington, MA, Reviewer: Vivek Sharma from Bombay, India. So each row can be obtianed by the Key value. Erik, you indicate that SQL Server loves unique indexes but this vendor solution seems to add (randomly in a lot of cases) unnecessary uniqueness by appending the PK (ID) column to the end of several indexes on any given table. Say, if am trying to make unique non clusterd index adding RowID in the non-clusterd index, BTree - RID + RowID (20+8) total of = 28 Bytes, Leaf - RID + TAID + Date + RowID (20+8+8+8) total of = 44 Bytes. I thought that ‘Scan’ counted the number of times the object was scanned. Kazakh / Қазақша If it is, please let us know via a Review, http://asktom.oracle.com/pls/asktom/f?p=100:11:::::P11_QUESTION_ID:3051796619914, http://asktom.oracle.com/pls/asktom/f?p=100:11:::::P11_QUESTION_ID:4941517587762, http://download-west.oracle.com/docs/cd/B10501_01/server.920/a96524/c11schem.htm. I understand that specifying uniqueness can serve to preserve integrity, but leaving that discussion aside for the moment, what are the perfomance consequences? Even though syntax are different the effect is the same. Scan count 1, logical reads, SQL SERVER – Performance Monitoring Tool – SQLGrease, SQL SERVER – Query Specific Wait Statistics and Performance Tuning, Is your SQL Server running slow and you want to speed it up without sharing server credentials? So it counts five, because SQL spawned a thread to manage parallelism. French / Français So, obviously going parallel threw some funk on the floor. You should see some of the Primary Keys people choose out there , You may have already read the CDC performance tuning white paper from MS, but just in case anyone stumbles across this who hasn’t, it may be worth a read: https://msdn.microsoft.com/en-us/library/dd266396%28v=sql.100%29.aspx?f=255&MSPPError=-2147217396, Thank you for sharing your thoughts and the URL with community. Every time I think I've got this nailed down it slips sideways a little bit on me. Let us see details about the statistics in the messages. Login to reply. However, after talking to a developer and senior architect, we realized that indexes can be unique. Catalan / Català The cluster should be unique, because of the need for a uniquifier if it isn't. If you had to spend all day flipping pages around, you’d probably be even lazier. The comparison between execution plans when I run the initial two SELECTs are identicial the only way I can get the Unique Table to use a Merge Join is to add a HINT of MAXDOP 1. I think it depends a bit on the data you’re indexing, and how cool your server is. I don't understand. If it’s large data loads, yes. Finnish / Suomi This make the index key larger and traversing the index tree a bit slower. You can also subscribe without commenting. My doubt is, even there would be a huge difference in the byte at btree, I can see an increase in Leaf node on modified index structure. All Rights Reserved. Performance Impact of Unique Index. The RID is the row identifier. He has authored 12 SQL Server database books, 35 Pluralsight courses and has written over 5200 articles on the database technology on his blog at a https://blog.sqlauthority.com. But! There are two non-clustered, non-unique indexes… I was just trying to convey like whats the significance of unique key while creating indexes. While the impact of deleted/updated PK’s key value(s) is unlikely, under normal circumstances, UNIQUE clustered and non-clustered indexes impact the BI side of data load process on the DWH server (in our case at least). Non-unique indexes are not used to enforce constraints on the tables with which they are associated. If I have a non-unique clustered index index and non-unique non-clusterd index indexthen my root level will have non-clustered key + clustered key + Uniquifier. Looking at the STATISTICS TIME and IO output, there’s not much difference in logical reads, but you see the non-unique index used all four cores available on my laptop (4 scans, 1 coordinator thread), and there’s a worktable and workfile for the hash join. There is a KB #302341(Last Review: 12/06/2015 03:30:31), which is not that old, it suggests to use trace flag 8207.
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