Cyril hahn dont save me mp3 full#

The behaviour of other arbitrary-result functions, like RAND(), is more likely to vary between DBMSs, so again check the documentation.Īlso I've seen ordering by UUID values being ignored in some contexts as the DB assumes that the type has no meaningful ordering. A function returning a new UUID should be evaluated every time in all DBMSs NEWID() is under MSSQL but it is worth verifying this in the documentation and/or by your own tests. The above is true for MSSQL (20 at least, and if I remember rightly 2000 as well). Ordering by NEWID() does work though, because if NEWID() was not always reassessed the purpose of UUIDs would be broken when inserting many new rows in one statemnt with unique identifiers as they key, so: SELECT display_name FROM tr_person ORDER BY NEWID()

The results are still ordered by the name indicating that the earlier sort field (the one expected to be random) has no effect so presumably always has the same value. To show that the same is the case with RAND() used in an ORDER BY clause, I try: SELECT display_name FROM tr_person ORDER BY RAND(), display_name The number does vary each time you run the query, but is the same for each row each time. Returns each name from our person table and a 'random' number, which is the same for each row. Pradeep Adiga's first suggestion, ORDER BY NEWID(), is fine and something I've used in the past for this reason.īe careful with using RAND() - in many contexts it is only executed once per statement so ORDER BY RAND() will have no effect (as you are getting the same result out of RAND() for each row).įor instance: SELECT display_name, RAND() FROM tr_person This should result in a stable sample selection.Īnyway, just thought this should be added to the discussion. There is also a REPEATABLE() option where you can pass a 'Seed' for page selection. So on that million row table doing top(20).TABLESAMPLE(20 PERCENT) the query drops to 5600 reads in 1600ms. Personally I've been using it to in effect limit the size of my table. The idea is you can make your ginormous table appear smaller prior to the expensive sort. So how do I use it? Select a subset size that more than covers the number of rows you need, then add a Top(). The actual number of rows you get back can vary based on what exists in the selected pages. SQL numbers each data page and selects X percent of those pages.

The idea behind table sample is to give you approximately the subset size you ask for. First an example: SELECT Top (20) *įROM Northwind.Orders TABLESAMPLE(20 PERCENT) I've only seen one article discussing it's use.there should be more. In SQL 2005 a new capability called TABLESAMPLE was created. So what's a boy to do? Meet TABLESAMPLE() The moral of the story is that if you have large tables (think billions of rows) or need to run this query frequently the newid() method breaks down. Running Order By newid() on this table generates 53,700 reads and takes 16 seconds.

Table B - has 1.2 million rows across 114,000 data pages.

The random query generates 145 reads in 42ms.

TableA - has 50,000 rows across 2500 data pages.

To give you a sense of how this scales, I'll give you two examples from a database I work with. Look at this typical execution plan, and note how the sort takes 96% of your time. If you've worked with any serious data volume this can rapidly become expensive.

Cyril hahn dont save me mp3 full#

The problem with this method, is that you're still guaranteed a full index scan and then a complete sort of the data. When someone get's fancy they add that you should really wrap NewID() in CheckSum(), you know, for performance! This is an old question, but one aspect of the discussion is missing, in my opinion - PERFORMANCE.