Press "Enter" to skip to content

Curated SQL Posts

Auto-Update Stats Threshold Change

Published 2016-03-03 by Kevin Feasel

Erik Darling points out that automatic statistics updates will happen more frequently in 2016 for large tables than in prior versions:

Here’s an abridged version of 10-20 million and 30-40 million rows, and how many modifications they took before a stats update occurred. If you follow the PercentMod column down, the returns diminish a bit the higher up you get. I’m not saying that I’d prefer to wait for 20% + 500 rows to modify, by any stretch. My only point here is that there’s not a set percentage to point to.

And, because you’re probably wondering, turning on Trace Flag 2371 in 2016 doesn’t make any difference.

This is a good change, though as Erik points out, if you’re managing very large tables, you might already have the trace flag on and thereby won’t see any difference.

Comments closed

Synonyms

Published 2016-03-03 by Kevin Feasel

Aaron Bertrand discusses synonyms:

Let’s say you have a table called dbo.BugReports, and you need to change it to dbo.SupportIncidents. This can be quite disruptive if you have references to the original name scattered throughout stored procedures, views, functions, and application code. Modern tools like SSDT can make a refactor relatively straightforward within the database (as long as queries aren’t constructed from user input and/or dynamic SQL), but for distributed applications, it can be a lot more complex.

A synonym can allow you to change the database now, and worry about the application later – even in phases. You just rename the table from the old name to the new name (or use ALTER TABLE ... SWITCH and then drop the original), and then create a synonym named with the old name that “points to” the new name

I’ve used synonyms once or twice, but they’re pretty low on my list, in part because of network effects:  if I create this great set of synonyms but the next guy doesn’t know about them, it makes maintenance that much harder.

Comments closed

Azure SQL Database Cross-Database Queries

Published 2016-03-03 by Kevin Feasel

James Serra reports that Azure SQL Database now allows cross-database queries:

A limitation with Azure SQL database has been its inability to do cross-database SQL queries.  This has changed with the introduction of elastic database queries, now in preview.  However, it’s not as easy as on-prem SQL Server, where you can just use the three-part name syntax DatabaseName.SchemaName.TableName.  Instead, you have to define remote tables (tables outside your current database), which is similar to how PolyBase works for those of you familiar with PolyBase.

For one-off tables you join to frequently, I suppose this isn’t terrible, but it’s certainly less convenient.

Comments closed

Backup Restore Chains

Published 2016-03-03 by Kevin Feasel

Anders Pedersen has a script to determine which backup files are needed to restore a database, but ran into a problem with LSNs matching:

Backup_set_id 1713355 and 1713378 has First_Lsn = Last_Lsn, and they are the same between the two records.  There was more records where this was true, just happens to be early in the result set so was easy to spot.

Question then arose in my mind if this caused a problem with the CTE, which it is obvious that it did, but to fix it, I would have to remove those records from being considered in the restore chain (or switch to a loop from a recursive CTE).

Check out the script.

Comments closed

Permissions To Create Stored Procedures

Published 2016-03-03 by Kevin Feasel

Kenneth Fisher shows the permissions necessary to create a stored procedure:

The user still won’t be able to create procedures or views. And if you think about it in the right way it makes sense. While the CREATE permissions give us the ability to create objects what they don’t give us is a place to put them. Schemas are containers for most of the objects in databases. So in order change the schema (by putting an object in it) we need to grant ALTER on it. So for the CREATE to work we need to:

Getting the right granularity for permissions is a vital part of securing a SQL Server instance.

Comments closed

Database File Info

Published 2016-03-03 by Kevin Feasel

Mike Fal shows us how to get database file information using Powershell:

To do this, I dove into the SMO object model. This gets a little /Net-y, but the good news is there’s lots of properties we can use to get the information we are looking for. If we look at both the DataFile and LogFile classes, there are properties readily available for us. Both classes have UsedSpace and Size properties (both measured in KB), from which we can derive both available space and percentage used. All it takes is wrapping some collection and formatting logic around these objects and we’re good to go. You can see my full function up on GitHub.

This is a nice example of using the Powershell pipeline to build an end product, in this case an HMTL report of log and file usage.

Comments closed

DBCC Extended Checks

Published 2016-03-02 by Kevin Feasel

The SQL Server CSS team has a new post on DBCC performance improvements:

Starting with SQL Server 2016, additional checks on filtered indexes, persisted computed columns, and UDT columns will not be run by default to avoid the expensive expression evaluation(s.)  This change greatly reduces the duration of CHECKDB against databases containing these objects.  However, the physical consistency checks of these objects is always completed.  Only when EXTENDED_LOGICAL_CHECKS option is specified will the expression evaluations be performed in addition to already present, logical checks (indexed view, XML indexes, and spatial indexes) as part of the EXTENDED_LOGICAL_CHECKS option.

 

For filtered indexes, CHECKDB has also been improved to skip records that do not qualify as being indexed by target NC index. 

This is telling me that we’ll want to have two separate CHECKDB processes, one which regularly runs CHECKDB (or CHECKTABLE) and one which occasionally runs CHECKDB with EXTENDED_LOGICAL_CHECKS.

Comments closed

Table Partitioning

Published 2016-03-02 by Kevin Feasel

Slava Murygin walks through partitioning:

Usually new Partition Ranges are added to the end of a Function/Schema, but in my example I’ll demonstrate how to split middle Partition.
At first we create new File Group. Then make it next within a schema and then split a Function:

All records, which are Less than 1/1/2015 and more than 1/1/2014, were moved to the new partition.

Partitioning is one of those topics I run into frequently enough to need to know it, but not frequently enough to memorize it; every time it feels like I’m starting from scratch.

Comments closed

Improving SMO Performance

Published 2016-03-02 by Kevin Feasel

Cody Konior shows how to use GetDefaultInitFields in SMO to improve performance, and also explains a bug when you try to do this:

It turns out has no default properties set, and the exception seems to occur after you try to set some. Interestingly though the way that boolean flag works is to set default properties on each SMO type; which means the solution is to turn it on for everything and then only disable it for this specific type; retaining any of the performance improvements throughout the rest of SMO.

Anything that makes SMO faster gets a vote of approval from me.

Comments closed

Using The YEAR Function

Published 2016-03-02 by Kevin Feasel

Kendra Little looks at the performance ramifications of using the YEAR() function:

But if we hover over that index seek, we can see in the tooltip that there’s a hidden predicate that is NOT a seek predicate. This is a hidden filter. And because this is SQL Server 2016, we can see “Number of Rows Read” — it had to read 9.3 million rows to count 1.9 million rows. It didn’t realize the 2006 rows were together– it checked all the females and examined the FakeBirthDateStamp column for each row.

Built-in functions can cause SARGability issues.

Comments closed