If you’ve ever needed to build a stored procedure that took a lot of different parameters and served a lot of different purposes, you’ve probably used dynamic SQL. It can be an awesome, powerful, and fast solution to a lot of performance problems. It can also be a route to sitting under your desk, banging your head against a wine bottle, wishing you could stop seeing double apostrophes mocking you.
In this session, I’ll:
- Build a multi-parameter stored procedure the old-fashioned way
- Show you why it doesn’t perform well
- Switch over to a dynamic SQL version
- Show you my favorite tips to make it faster and easier to troubleshoot (and note that in the live class, I don’t cover all of the tips – the script keeps going with my conversational style in the comments, so you can read through more tips & tricks after class)
I’ll be using the Stack Overflow public database. You can use any size, small/medium/large – they’ll all work to show the problems.
After this session, your next steps for learning are:
- What Is SQL Injection? by Brent Ozar
- 4 SQL Injection Techniques for Stealing Data by Bert Wagner
- The Curse and Blessings of Dynamic SQL by Erland Sommarskog
- Dynamic Search Conditions in T-SQL by Erland Sommarskog
- Video: Dynamic SQL Pro Tips – from my Mastering Query Tuning class, where I go into even more details
Here’s the demo script to follow along:
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/* Dynamic SQL - When, Why, and How to Use It v2.1 - September 2023 You're comfortable writing T-SQL, and you've built a lot of stored procedures that have a bunch of parameters. For example, you have that "product search" stored proc with parameters for product category, name, price range, sort order, etc, and you have to accept any of 'em. So how do we make those go fast? And how can we get 'em to use indexes? In one all-demo hour, you'll learn how to write dynamic SQL that's easy to tune, manage, and troubleshoot. Download the latest version free: https://www.BrentOzar.com/go/dynamicsql Open source with the Creative Commons License. For details, see the end of this file. Requirements: * Any Stack Overflow database: https://BrentOzar.com/go/querystack * Any supported SQL Server or Azure SQL DB This first RAISERROR is just to make sure you don't accidentally hit F5 and run the entire script. You don't need to run this: */ RAISERROR(N'Oops! No, don''t just hit F5. Run these demos one at a time.', 20, 1) WITH LOG; GO /* Set up indexes for the demo: */ EXEC DropIndexes; GO CREATE INDEX DisplayName ON dbo.Users(DisplayName); CREATE INDEX Location ON dbo.Users(Location); CREATE INDEX Reputation ON dbo.Users(Reputation); GO /* Turn on our tuning options and Actual Execution Plans: */ SET STATISTICS IO ON; GO /* Meet the Users table: */ SELECT TOP 100 * FROM dbo.Users; GO /* How big is it? Look at how many rows are returned from this query, and on the Messages tab, check the STATISTICS IO results to see how many logical reads we did. That's how many 8KB pages the clustered index has. */ SET STATISTICS IO ON; SELECT COUNT(*) FROM dbo.Users WITH (INDEX = 1); /* The clustered index */ GO /* Folks want a single stored procedure that can search on any of these 3 fields. You should be able to pass in 1, 2, or all 3, and find just the users that match ALL of your parameters. They want a stored proc that works like this: */ EXEC usp_SearchUsers @SearchDisplayName = 'Brent%'; GO EXEC usp_SearchUsers @SearchLocation = 'Indiana%'; GO EXEC usp_SearchUsers @SearchReputation = 2; GO EXEC usp_SearchUsers @SearchDisplayName = 'Brent%', @SearchLocation = 'Las Vegas%'; GO EXEC usp_SearchUsers @SearchLocation = 'Indiana%', @SearchReputation = 107548; GO /* Here's one way to do it. It's a bad way - it doesn't really work - but let's look at how it performs: */ CREATE OR ALTER PROC dbo.usp_SearchUsers @SearchDisplayName NVARCHAR(100) = NULL, @SearchLocation NVARCHAR(100) = NULL, @SearchReputation INT = NULL AS BEGIN IF @SearchDisplayName IS NOT NULL SELECT * FROM dbo.Users WHERE DisplayName LIKE @SearchDisplayName ORDER BY CreationDate; ELSE IF @SearchLocation IS NOT NULL SELECT * FROM dbo.Users WHERE Location LIKE @SearchLocation ORDER BY CreationDate; ELSE IF @SearchReputation IS NOT NULL SELECT * FROM dbo.Users WHERE Reputation = @SearchReputation ORDER BY CreationDate; END GO /* Will that work? Is there a bug in that logic? */ /* If we pass in multiple fields, like this, it only searches for one of them: */ EXEC usp_SearchUsers @SearchDisplayName = 'Brent%', @SearchReputation = 1; GO /* Do other parameters use indexes? */ EXEC usp_SearchUsers @SearchLocation = 'Indiana%'; GO /* When I'm looking at search code performance, I'll often ask: * Is the optimizer willing to use indexes? * Are the row estimates right? In this case, all 3 of the branches were willing to use indexes - too willing - and the estimates weren't right - because SQL Server built the entire execution plan for all 3 branches when we ran the very first query. Notice some of the query plans have yellow bang warnings on their sort operator because SQL Server underestimated how many rows those queries would bring back. It's that pesky ORDER BY CreationDate. Does it get better if you clear the cache, and try a different branch first? */ DBCC FREEPROCCACHE; GO EXEC usp_SearchUsers @SearchLocation = 'Indiana%'; GO EXEC usp_SearchUsers @SearchReputation = 2; GO EXEC usp_SearchUsers @SearchDisplayName = 'Brent%'; GO /* Sometimes - for some branches - but not always. That's the devil of parameter sniffing - the whole plan gets optimized for the first set of parameters that get used, even if some branches rely on parameters that didn't get passed in. They can get optimized for NULL, which can give you really low estimates. Sometimes that's good - but not usually. The IF branch design has a few drawbacks: * It's very susceptible to parameter sniffing * Multi-parameter results don't match what the users asked for * Getting multi-parameter results to work right would require a lot of code IF @SearchDisplayName IS NOT NULL AND @SearchLocation IS NOT NULL ... IF @SearchDisplayName IS NOT NULL AND @SearchLocation IS NOT NULL AND @SearchReputation.... ... Yeah, that's not gonna scale. So let's try a different design: */ CREATE OR ALTER PROC dbo.usp_SearchUsers @SearchDisplayName NVARCHAR(100) = NULL, @SearchLocation NVARCHAR(100) = NULL, @SearchReputation INT = NULL AS BEGIN SELECT * FROM dbo.Users WHERE (DisplayName LIKE @SearchDisplayName OR @SearchDisplayName IS NULL) AND (Location LIKE @SearchLocation OR @SearchLocation IS NULL) AND (Reputation = @SearchReputation OR @SearchReputation IS NULL) ORDER BY CreationDate; END GO EXEC usp_SearchUsers @SearchDisplayName = 'Brent%', @SearchReputation = 1; GO /* Do other parameters use indexes? */ EXEC usp_SearchUsers @SearchLocation = 'Indiana%'; GO /* Look at the plans, and... HAHAHAHAHAHAHAHAHAHAHAHA (breathes) HAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHA Then look at logical reads! (passes out) That, ladies and gentlemen, is a really bad query plan. SQL Server uses the first set of incoming parameters to build the plan - in our case, it builds a plan that uses the DisplayName index. And when DisplayName isn't passed in, SQL SERVER STILL USES THAT INDEX, doing a key lookup of every single row. It does millions of logical reads - even though the whole table only has about 80k pages. Alright, screw that. How about ISNULL? */ CREATE OR ALTER PROC dbo.usp_SearchUsers @SearchDisplayName NVARCHAR(100) = NULL, @SearchLocation NVARCHAR(100) = NULL, @SearchReputation INT = NULL AS BEGIN SELECT * FROM dbo.Users WHERE DisplayName LIKE ISNULL(@SearchDisplayName, DisplayName) AND Location LIKE ISNULL(@SearchLocation, Location) AND Reputation = ISNULL(@SearchReputation, Reputation) ORDER BY CreationDate; END GO EXEC usp_SearchUsers @SearchDisplayName = 'Brent%', @SearchReputation = 1; GO /* Do other parameters use indexes? */ EXEC usp_SearchUsers @SearchLocation = 'Indiana%'; GO /* How about COALESCE? */ CREATE OR ALTER PROC dbo.usp_SearchUsers @SearchDisplayName NVARCHAR(100) = NULL, @SearchLocation NVARCHAR(100) = NULL, @SearchReputation INT = NULL, @OrderBy NVARCHAR(100) = 'CreationDate' AS BEGIN SELECT * FROM dbo.Users WHERE DisplayName LIKE COALESCE(@SearchDisplayName, DisplayName) AND Location LIKE COALESCE(@SearchLocation, Location) AND Reputation = COALESCE(@SearchReputation, Reputation) ORDER BY CreationDate; END GO EXEC usp_SearchUsers @SearchDisplayName = 'Brent%', @SearchReputation = 1; GO /* Do other parameters use indexes? */ EXEC usp_SearchUsers @SearchLocation = 'Indiana%'; GO /* Depending on your version & cardinality estimator, you're probably going to get either clustered index scans, or some very unusual index intersection plans that don't perform worth a dang. Now you see why people have to resort to dynamic SQL. So let's spend some time at the Dynamic SQL Resort: */ CREATE OR ALTER PROC dbo.usp_SearchUsers @SearchDisplayName NVARCHAR(100) = NULL, @SearchLocation NVARCHAR(100) = NULL, @SearchReputation INT = NULL AS BEGIN DECLARE @StringToExecute NVARCHAR(4000); SET @StringToExecute = N'SELECT * FROM dbo.Users WHERE 1 = 1 '; IF @SearchDisplayName IS NOT NULL SET @StringToExecute = @StringToExecute + N' AND DisplayName LIKE @SearchDisplayName '; IF @SearchLocation IS NOT NULL SET @StringToExecute = @StringToExecute + N' AND Location LIKE @SearchLocation '; IF @SearchReputation IS NOT NULL SET @StringToExecute = @StringToExecute + N' AND Reputation = @SearchReputation '; SET @StringToExecute = @StringToExecute + N' ORDER BY CreationDate; '; EXEC sp_executesql @StringToExecute, N'@SearchDisplayName NVARCHAR(100), @SearchLocation NVARCHAR(100), @SearchReputation INT', @SearchDisplayName, @SearchLocation, @SearchReputation; END GO EXEC usp_SearchUsers @SearchDisplayName = 'Brent%', @SearchReputation = 1; GO /* Do other parameters use indexes? */ EXEC usp_SearchUsers @SearchLocation = 'Indiana%'; GO /* Is it bloating the plan cache? */ DBCC FREEPROCCACHE GO EXEC usp_SearchUsers @SearchDisplayName = 'Brent%'; GO EXEC usp_SearchUsers @SearchLocation = 'Indiana%'; GO EXEC usp_SearchUsers @SearchReputation = 2; GO EXEC usp_SearchUsers @SearchDisplayName = 'Brent%', @SearchLocation = 'Las Vegas%'; GO EXEC usp_SearchUsers @SearchLocation = 'Indiana%', @SearchReputation = 89836; GO EXEC usp_SearchUsers @SearchDisplayName = 'Brent%', @SearchReputation = 12145; GO sp_BlitzCache GO /* There's a line in the plan cache for every variation of the dynamic SQL, but whenever we use different parameters, they'll reuse an existing plan. This might look like a lot of plans at first glance - but it's not really that many, especially as opposed to building a line for every single combination of parameter VALUES, like Brent or Alex or Billie. It's a little bit of plan cache bloat - but totally worth it since each combination of parameters can get its own customized plan with good (hopefully) indexes and memory grants. We do still have parameter sniffing: */ EXEC usp_SearchUsers @SearchLocation = 'Indiana%'; EXEC usp_SearchUsers @SearchLocation = 'India%'; GO /* Now for the pro tips. Lemme clear the cache and run it a couple of times first: */ DBCC FREEPROCCACHE; GO EXEC usp_SearchUsers @SearchDisplayName = 'Brent%', @SearchLocation = 'Las Vegas%'; GO 5 sp_BlitzCache; GO /* Pro Tip: in your dynamic SQL, throw in a comment to include the source. */ CREATE OR ALTER PROC dbo.usp_SearchUsers @SearchDisplayName NVARCHAR(100) = NULL, @SearchLocation NVARCHAR(100) = NULL, @SearchReputation INT = NULL AS BEGIN DECLARE @StringToExecute NVARCHAR(4000); SET @StringToExecute = N'SELECT /* usp_SearchUsers */ * FROM dbo.Users WHERE 1 = 1 '; IF @SearchDisplayName IS NOT NULL SET @StringToExecute = @StringToExecute + N' AND DisplayName LIKE @SearchDisplayName '; IF @SearchLocation IS NOT NULL SET @StringToExecute = @StringToExecute + N' AND Location LIKE @SearchLocation '; IF @SearchReputation IS NOT NULL SET @StringToExecute = @StringToExecute + N' AND Reputation = @SearchReputation '; SET @StringToExecute = @StringToExecute + N' ORDER BY CreationDate; '; EXEC sp_executesql @StringToExecute, N'@SearchDisplayName NVARCHAR(100), @SearchLocation NVARCHAR(100), @SearchReputation INT', @SearchDisplayName, @SearchLocation, @SearchReputation; END GO /* So that when you're viewing plans, you can see the source. */ */ DBCC FREEPROCCACHE; GO EXEC usp_SearchUsers @SearchDisplayName = 'Brent%'; GO sp_BlitzCache; GO /* Pro Tip: add line feeds to make it easier to read. */ CREATE OR ALTER PROC dbo.usp_SearchUsers @SearchDisplayName NVARCHAR(100) = NULL, @SearchLocation NVARCHAR(100) = NULL, @SearchReputation INT = NULL, @OrderBy NVARCHAR(100) = 'CreationDate' AS BEGIN DECLARE @StringToExecute NVARCHAR(4000); DECLARE @crlf NVARCHAR(2) = NCHAR(13) + NCHAR(10); SET @StringToExecute = @crlf + N' SELECT /* usp_SearchUsers */ * FROM dbo.Users WHERE 1 = 1 ' + @crlf; IF @SearchDisplayName IS NOT NULL SET @StringToExecute = @StringToExecute + N' AND DisplayName LIKE @SearchDisplayName ' + @crlf; IF @SearchLocation IS NOT NULL SET @StringToExecute = @StringToExecute + N' AND Location LIKE @SearchLocation ' + @crlf; IF @SearchReputation IS NOT NULL SET @StringToExecute = @StringToExecute + N' AND Reputation = @SearchReputation ' + @crlf; SET @StringToExecute = @StringToExecute + N' ORDER BY CreationDate; '; EXEC sp_executesql @StringToExecute, N'@SearchDisplayName NVARCHAR(100), @SearchLocation NVARCHAR(100), @SearchReputation INT', @SearchDisplayName, @SearchLocation, @SearchReputation; END GO /* Now check out the nicely formatted string - after sp_BlitzCache runs, click on the query plan, right-click on the query, and click Edit Query Text: */ DBCC FREEPROCCACHE; GO EXEC usp_SearchUsers @SearchDisplayName = 'Brent%'; GO sp_BlitzCache; GO /* But do NOT put dynamic stuff in the comments, les you end up with different plans for every dynamic thing: */ CREATE OR ALTER PROC dbo.usp_SearchUsers @SearchDisplayName NVARCHAR(100) = NULL, @SearchLocation NVARCHAR(100) = NULL, @SearchReputation INT = NULL, @OrderBy NVARCHAR(100) = 'CreationDate' AS BEGIN DECLARE @StringToExecute NVARCHAR(4000); DECLARE @crlf NVARCHAR(2) = NCHAR(13) + NCHAR(10); SET @StringToExecute = @crlf + N'SELECT /* usp_SearchUsers at date/time: ' + CONVERT(NVARCHAR(100), GETDATE(), 21) + '*/' + @crlf + N' * FROM dbo.Users WHERE 1 = 1 ' + @crlf; IF @SearchDisplayName IS NOT NULL SET @StringToExecute = @StringToExecute + N' AND DisplayName LIKE @SearchDisplayName ' + @crlf; IF @SearchLocation IS NOT NULL SET @StringToExecute = @StringToExecute + N' AND Location LIKE @SearchLocation ' + @crlf; IF @SearchReputation IS NOT NULL SET @StringToExecute = @StringToExecute + N' AND Reputation = @SearchReputation ' + @crlf; SET @StringToExecute = @StringToExecute + N' ORDER BY CreationDate; '; EXEC sp_executesql @StringToExecute, N'@SearchDisplayName NVARCHAR(100), @SearchLocation NVARCHAR(100), @SearchReputation INT', @SearchDisplayName, @SearchLocation, @SearchReputation; END GO /* Because every string will be unique: */ DBCC FREEPROCCACHE; GO EXEC usp_SearchUsers @SearchDisplayName = 'Brent%'; GO EXEC usp_SearchUsers @SearchDisplayName = 'Brent%'; GO EXEC usp_SearchUsers @SearchDisplayName = 'Brent%'; GO sp_BlitzCache; GO /* See how sp_BlitzCache shows four different lines of dynamic SQL now? Click on each of their plans, and look at the query text to see how the time text comments meant that each query has different text, so it gets different entries in the plan cache. */ /* Pro Tip: Every proc with dynamic SQL needs a @Debug_PrintQuery parameter to print strings: */ CREATE OR ALTER PROC dbo.usp_SearchUsers @SearchDisplayName NVARCHAR(100) = NULL, @SearchLocation NVARCHAR(100) = NULL, @SearchReputation INT = NULL, @Debug_PrintQuery TINYINT = 0, @Debug_ExecuteQuery TINYINT = 1 AS BEGIN DECLARE @StringToExecute NVARCHAR(4000); DECLARE @crlf NVARCHAR(2) = NCHAR(13) + NCHAR(10); SET @StringToExecute = @crlf + N'SELECT /* usp_SearchUsers */ * FROM dbo.Users WHERE 1 = 1 ' + @crlf; IF @SearchDisplayName IS NOT NULL SET @StringToExecute = @StringToExecute + N' AND DisplayName LIKE @SearchDisplayName ' + @crlf; IF @SearchLocation IS NOT NULL SET @StringToExecute = @StringToExecute + N' AND Location LIKE @SearchLocation ' + @crlf; IF @SearchReputation IS NOT NULL SET @StringToExecute = @StringToExecute + N' AND Reputation = @SearchReputation ' + @crlf; SET @StringToExecute = @StringToExecute + N' ORDER BY CreationDate; '; IF @Debug_PrintQuery = 1 PRINT @StringToExecute; IF @Debug_ExecuteQuery = 1 EXEC sp_executesql @StringToExecute, N'@SearchDisplayName NVARCHAR(100), @SearchLocation NVARCHAR(100), @SearchReputation INT', @SearchDisplayName, @SearchLocation, @SearchReputation; END GO /* So you can see the T-SQL in the Messages tab, complete with line returns: */ EXEC usp_SearchUsers @SearchDisplayName = 'Brent%', @Debug_PrintQuery = 1, @Debug_ExecuteQuery = 1; GO /* PRINT is limited to 4,000 characters. If you build longer strings, use this stored procedure, Helper_LongPrint: https://www.codeproject.com/Articles/18881/SQL-String-Printing */ /* Let's implement that ORDER BY statement. Just to keep the projector screen readable, I'm going to ignore the filters for @SearchLocation and @SearchReputation, and only work on the ORDER BY part. */ CREATE OR ALTER PROC dbo.usp_SearchUsers @SearchDisplayName NVARCHAR(100) = NULL, @OrderBy NVARCHAR(100) = 'CreationDate', @Debug_PrintQuery TINYINT = 0 AS BEGIN DECLARE @StringToExecute NVARCHAR(4000); DECLARE @crlf NVARCHAR(2) = NCHAR(13) + NCHAR(10); SET @StringToExecute = @crlf + N'SELECT /* usp_SearchUsers */ * FROM dbo.Users WHERE 1 = 1 ' + @crlf; IF @SearchDisplayName IS NOT NULL SET @StringToExecute = @StringToExecute + N' AND DisplayName LIKE @SearchDisplayName ' + @crlf; IF @OrderBy IS NOT NULL SET @StringToExecute = @StringToExecute + N' ORDER BY ' + @OrderBy; IF @Debug_PrintQuery = 1 PRINT @StringToExecute; EXEC sp_executesql @StringToExecute, N'@SearchDisplayName NVARCHAR(100)', @SearchDisplayName; END GO /* Let's see what QUERY (not query plan) that it builds. I'm less worried about performance here, and more just worried about getting the query right first. */ SET STATISTICS IO, TIME OFF; GO EXEC usp_SearchUsers @SearchDisplayName = 'Brent%', @OrderBy = 'Reputation', @Debug_PrintQuery = 1; GO /* And try descending order: */ EXEC usp_SearchUsers @SearchDisplayName = 'Brent%', @OrderBy = 'Reputation DESC', @Debug_PrintQuery = 1; GO /* Pretty cool. And how about this: */ EXEC usp_SearchUsers @SearchDisplayName = 'Brent%', @OrderBy = 'Reputation DESC; SELECT * FROM sys.databases;', @Debug_PrintQuery = 1; GO /* Oh. Well, that's not good. That's SQL injection. Never, ever use the stored procedure's input parameters as part of the dynamic SQL itself. Never, ever. Obligatory comic strip: https://xkcd.com/327/ Think you can work around it via regex and sanitizing? Think again, as Bert Wagner explains: https://bertwagner.com/posts/4-sql-injection-techniques-for-stealing-data/ Instead, examine the user's input, and substitute your own: */ CREATE OR ALTER PROC dbo.usp_SearchUsers @SearchDisplayName NVARCHAR(100) = NULL, @OrderBy NVARCHAR(100) = 'CreationDate', @Debug_PrintQuery TINYINT = 0 AS BEGIN DECLARE @StringToExecute NVARCHAR(4000); DECLARE @crlf NVARCHAR(2) = NCHAR(13) + NCHAR(10); SET @StringToExecute = @crlf + N'SELECT /* usp_SearchUsers */ * FROM dbo.Users u WHERE 1 = 1 ' + @crlf; IF @SearchDisplayName IS NOT NULL SET @StringToExecute = @StringToExecute + N' AND DisplayName LIKE @SearchDisplayName ' + @crlf; IF @OrderBy IS NOT NULL BEGIN SET @StringToExecute = @StringToExecute + N' ORDER BY '; SET @StringToExecute = @StringToExecute + CASE WHEN @OrderBy LIKE 'CreationDate%' THEN N' u.CreationDate ' WHEN @OrderBy LIKE 'DisplayName%' THEN N' u.DisplayName ' WHEN @OrderBy LIKE 'Location%' THEN N' u.Location ' WHEN @OrderBy LIKE 'Reputation%' THEN N' u.Reputation ' ELSE N' u.Id ' /* Or whatever default ordering you want, ideally to make SQL's life easier */ END; IF @OrderBy LIKE '% DESC' SET @StringToExecute = @StringToExecute + N' DESC '; SET @StringToExecute = @StringToExecute + N';'; /* Because technically, we're supposed to */ END IF @Debug_PrintQuery = 1 PRINT @StringToExecute; EXEC sp_executesql @StringToExecute, N'@SearchDisplayName NVARCHAR(100), @OrderBy NVARCHAR(100)', @SearchDisplayName, @OrderBy; END GO /* Test it: */ EXEC usp_SearchUsers @SearchDisplayName = 'Brent%', @OrderBy = 'Reputation', @Debug_PrintQuery = 1; EXEC usp_SearchUsers @SearchDisplayName = 'Brent%', @OrderBy = 'Reputation DESC', @Debug_PrintQuery = 1; EXEC usp_SearchUsers @SearchDisplayName = 'Brent%', @OrderBy = 'Reputation DESC; SELECT * FROM sys.databases;', @Debug_PrintQuery = 1; GO /* Note that the more complex our dynamic SQL gets, like with different possible filters, joins, and order by clauses, the more we may need specialized indexes. But don't try to guess which ones need indexes! Use sp_BlitzCache to review your most resource-intensive queries. That'll tell you which ones need the most help because they're getting called more frequently and/or sucking the most. What if you want to order by MULTIPLE columns? If you pass in a list, then whatever method you use to split the string list has to preserve the same order. Unfortunately, SPLIT_STRING doesn't: it doesn't guarantee that the output will be in the same order. For more info: https://feedback.azure.com/forums/908035-sql-server/suggestions/32902852-string-split-is-not-feature-complete Instead, we'll either have to use our own string splitter, or pass in a table. I'm going to use a splitter function that's widely regarded as one of the fastest ones out there, Jeff Moden's: https://www.sqlservercentral.com/articles/tally-oh-an-improved-sql-8k-%e2%80%9ccsv-splitter%e2%80%9d-function Create the function. Note that I'm using the name STRING_SPLIT, which is the same as a system function, but there's a reason: the internal function doesn't guarantee order and doesn't include a row number. To learn more: https://www.brentozar.com/archive/2016/03/splitting-strings-sql-server-2016-rescue/ */ CREATE OR ALTER FUNCTION [dbo].[STRING_SPLIT] (@pString VARCHAR(8000), @pDelimiter CHAR(1)) RETURNS TABLE WITH SCHEMABINDING AS RETURN WITH E1(N) AS ( SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 ), --10E+1 or 10 rows E2(N) AS (SELECT 1 FROM E1 a, E1 b), --10E+2 or 100 rows E4(N) AS (SELECT 1 FROM E2 a, E2 b), --10E+4 or 10,000 rows max cteTally(N) AS (--==== This provides the "base" CTE and limits the number of rows right up front -- for both a performance gain and prevention of accidental "overruns" SELECT TOP (ISNULL(DATALENGTH(@pString),0)) ROW_NUMBER() OVER (ORDER BY (SELECT NULL)) FROM E4 ), cteStart(N1) AS (--==== This returns N+1 (starting position of each "element" just once for each delimiter) SELECT 1 UNION ALL SELECT t.N+1 FROM cteTally t WHERE SUBSTRING(@pString,t.N,1) = @pDelimiter ), cteLen(N1,L1) AS(--==== Return start and length (for use in substring) SELECT s.N1, ISNULL(NULLIF(CHARINDEX(@pDelimiter,@pString,s.N1),0)-s.N1,8000) FROM cteStart s ) --===== Do the actual split. The ISNULL/NULLIF combo handles the length for the final element when no delimiter is found. SELECT ItemNumber = ROW_NUMBER() OVER(ORDER BY l.N1), Item = SUBSTRING(@pString, l.N1, l.L1) FROM cteLen l ; GO /* When you call it, here's what you get: */ SELECT * FROM dbo.STRING_SPLIT('Location, DisplayName DESC, Reputation', ',') ORDER BY ItemNumber; GO /* Note that there are spaces we'll need to clean up. */ CREATE OR ALTER PROC dbo.usp_SearchUsers @SearchDisplayName NVARCHAR(100) = NULL, @OrderBy VARCHAR(4000) = 'CreationDate', /* This can now be a comma-delimited list of columns */ @Debug_PrintQuery TINYINT = 0 AS BEGIN DECLARE @StringToExecute NVARCHAR(4000), @OrderByItemNumber INT, @OrderByItem VARCHAR(100); DECLARE @crlf NVARCHAR(2) = NCHAR(13) + NCHAR(10); SET @StringToExecute = @crlf + N'/* usp_SearchUsers */' + @crlf + N' SELECT * FROM dbo.Users u WHERE 1 = 1 ' + @crlf; IF @SearchDisplayName IS NOT NULL SET @StringToExecute = @StringToExecute + N' AND DisplayName LIKE @SearchDisplayName ' + @crlf; IF @OrderBy IS NOT NULL BEGIN SET @StringToExecute = @StringToExecute + N' ORDER BY '; /* Split the ordering string */ DECLARE cursor_OrderBy CURSOR FOR SELECT ItemNumber, RTRIM(LTRIM(Item)) FROM dbo.STRING_SPLIT(@OrderBy, ',') ORDER BY ItemNumber; OPEN cursor_OrderBy; FETCH NEXT FROM cursor_OrderBy INTO @OrderByItemNumber, @OrderByItem; WHILE @@FETCH_STATUS = 0 BEGIN IF @OrderByItemNumber > 1 SET @StringToExecute = @StringToExecute + ' , '; SET @StringToExecute = @StringToExecute + CASE WHEN @OrderByItem LIKE 'CreationDate%' THEN N' u.CreationDate ' WHEN @OrderByItem LIKE 'DisplayName%' THEN N' u.DisplayName ' WHEN @OrderByItem LIKE 'Location%' THEN N' u.Location ' WHEN @OrderByItem LIKE 'Reputation%' THEN N' u.Reputation ' ELSE N' u.Id ' /* Or whatever default ordering you want, ideally to make SQL's life easier */ END; IF @OrderByItem LIKE '% DESC' SET @StringToExecute = @StringToExecute + N' DESC '; FETCH NEXT FROM cursor_OrderBy INTO @OrderByItemNumber, @OrderByItem; END /* Cursor */ CLOSE cursor_OrderBy; DEALLOCATE cursor_OrderBy; SET @StringToExecute = @StringToExecute + N';'; /* Because technically, we're supposed to */ END IF @Debug_PrintQuery = 1 PRINT @StringToExecute; EXEC sp_executesql @StringToExecute, N'@SearchDisplayName NVARCHAR(100), @OrderBy NVARCHAR(100)', @SearchDisplayName, @OrderBy; END GO EXEC usp_SearchUsers @SearchDisplayName = 'Brent Ozar', @OrderBy = 'Reputation DESC, DisplayName, Location', @Debug_PrintQuery = 1; /* Next up, how about letting users pick what fields they want to see, and from which tables? We already know we NEVER EVER want to let them send their own T-SQL in as a parameter because it's a recipe for SQL injection. Here's one implementation - again, I'm going to remove the search parameters and the order-by stuff just to keep my presentation screen easier to read, and only focus on the dynamic join implementation: */ CREATE OR ALTER PROC dbo.usp_SearchUsers @SearchDisplayName NVARCHAR(100) = NULL, @ShowTotalVotes BIT = 0, @Debug_PrintQuery TINYINT = 0 AS BEGIN DECLARE @StringToExecute NVARCHAR(4000); DECLARE @crlf NVARCHAR(2) = NCHAR(13) + NCHAR(10); SET @StringToExecute = @crlf + N'/* usp_SearchUsers */' + @crlf + N' SELECT * FROM dbo.Users u WHERE 1 = 1 ' + @crlf; IF @SearchDisplayName IS NOT NULL SET @StringToExecute = @StringToExecute + N' AND DisplayName LIKE @SearchDisplayName ' + @crlf; IF @ShowTotalVotes = 1 SET @StringToExecute = @StringToExecute + N' LEFT OUTER JOIN dbo.Votes v ON u.Id = v.UserId '; IF @Debug_PrintQuery = 1 PRINT @StringToExecute; EXEC sp_executesql @StringToExecute, N'@SearchDisplayName NVARCHAR(100)', @SearchDisplayName; END GO /* Let's see how it works: */ EXEC usp_SearchUsers @SearchDisplayName = 'Brent%', @ShowTotalVotes = 1, @Debug_PrintQuery = 1; GO /* Well, that's not gonna work. Our dynamic SQL building is gonna have to get a lot more intelligent, and we'll need to build the string in different parts: * @Select * @From * @Where * @OrderBy And then at the end of our string-building, assemble them all into one like some kind of transformer. */ CREATE OR ALTER PROC dbo.usp_SearchUsers @SearchDisplayName NVARCHAR(100) = NULL, @ShowTotalVotes BIT = 0, @Debug_PrintQuery TINYINT = 0 AS BEGIN DECLARE @StringToExecute NVARCHAR(4000), @Select NVARCHAR(4000), @From NVARCHAR(4000), @Where NVARCHAR(4000); DECLARE @crlf NVARCHAR(2) = NCHAR(13) + NCHAR(10); SET @Select = @crlf + N'/* usp_SearchUsers */' + @crlf + N' SELECT u.Id, u.DisplayName, u.Location ' + @crlf; SET @From = N' FROM dbo.Users u ' + @crlf; SET @Where = N' WHERE 1 = 1 ' + @crlf; IF @SearchDisplayName IS NOT NULL SET @Where = @Where + N' AND DisplayName LIKE @SearchDisplayName ' + @crlf; IF @ShowTotalVotes = 1 BEGIN SET @Select = @Select + N' , VotesCast = (SELECT SUM(1) FROM dbo.Votes v WHERE u.Id = v.UserId) ' + @crlf; END /* Autobots, unite! */ SET @StringToExecute = @Select + @From + @Where; IF @Debug_PrintQuery = 1 PRINT @StringToExecute; EXEC sp_executesql @StringToExecute, N'@SearchDisplayName NVARCHAR(100)', @SearchDisplayName; END GO /* Let's see how it works: */ EXEC usp_SearchUsers @SearchDisplayName = 'Brent%', @ShowTotalVotes = 0, @Debug_PrintQuery = 1; EXEC usp_SearchUsers @SearchDisplayName = 'Brent%', @ShowTotalVotes = 1, @Debug_PrintQuery = 1; GO /* The beauty of dynamic SQL here is that you're only joining out to the Votes table if you really need to show its contents. Otherwise, you keep the query fast by only looking at the Users table. You can also leverage views here to put the optional joins and fields in the view, and SQL Server will (usually) do a good job of eliminating unneccessary joins, but that also means you can't just SELECT *. You have to be really careful about only selecting the fields you need. */ /* Debugging this stuff can be AWFUL, especially when you start nesting dynamic SQL inside of dynamic SQL. It can even be tough figuring out which part of your query is throwing the error - normal SQL, or dynamic SQL, since the line numbers are worthless too. Run this, and try to figure out where the error is: */ CREATE OR ALTER PROC dbo.usp_SearchUsers @SearchDisplayName NVARCHAR(100) = NULL, @ShowTotalVotes BIT = 0, @Debug_PrintQuery TINYINT = 0 AS BEGIN DECLARE @StringToExecute NVARCHAR(4000), @Select NVARCHAR(4000), @From NVARCHAR(4000), @Where NVARCHAR(4000); DECLARE @crlf NVARCHAR(2) = NCHAR(13) + NCHAR(10); SET @Select = @crlf + N'SELECT /* usp_SearchUsers */ u.Id, u.DisplayName, u.Location ' + @crlf; SET @From = N' FROM dbo.Users u ' + @crlf; SET @Where = N' WHERE ' + @crlf; IF @SearchDisplayName IS NOT NULL SET @Where = @Where + N' AND DisplayName LIKE @SearchDisplayName ' + @crlf; IF @ShowTotalVotes = 1 BEGIN SET @Select = @Select + N' , VotesCast = (SELECT SUM(1) FROM dbo.Votes v WHERE u.Id = v.UserId) ' + @crlf; END /* Autobots, unite! */ SET @StringToExecute = @Select + @From + @Where; IF @Debug_PrintQuery = 1 PRINT @StringToExecute; EXEC sp_executesql @StringToExecute, N'@SearchDisplayName NVARCHAR(100)', @SearchDisplayName; END GO /* Test it: */ EXEC usp_SearchUsers @SearchDisplayName = 'Brent%', @ShowTotalVotes = 0 GO /* OMG THIS IS TERRIBLE And that line number is worthless. Is that "AND" inside our regular query, or the dynamic SQL? So when you build dynamic SQL, it can help to use different casing for your keywords and variable names inside dynamic SQL, like this: Regular query: */ DECLARE @MyVariable INT = 1; SELECT DisplayName FROM dbo.Users u WHERE Id = @MyVariable AND 1 = 1; GO /* Inside my dynamic SQL: */ declare @myvariable int = 1; select displayname from dbo.users U where id = @myvariable and 1 = 1; GO /* Let's change the casing on our stored procedure: */ CREATE OR ALTER PROC dbo.usp_SearchUsers @SearchDisplayName NVARCHAR(100) = NULL, @ShowTotalVotes BIT = 0, @Debug_PrintQuery TINYINT = 0 AS BEGIN DECLARE @StringToExecute NVARCHAR(4000), @Select NVARCHAR(4000), @From NVARCHAR(4000), @Where NVARCHAR(4000); DECLARE @crlf NVARCHAR(2) = NCHAR(13) + NCHAR(10); SET @Select = @crlf + N'select /* usp_SearchUsers */ U.id, U.displayname, U.location ' + @crlf; SET @From = N' from dbo.users U ' + @crlf; SET @Where = N' where ' + @crlf; IF @SearchDisplayName IS NOT NULL SET @Where = @Where + N' and U.displayname like @searchdisplayname ' + @crlf; IF @ShowTotalVotes = 1 BEGIN SET @Select = @Select + N' , VotesCast = (select sum(1) from dbo.votes V where U.id = V.userid) ' + @crlf; END /* Autobots, unite! */ SET @StringToExecute = @Select + @From + @Where; IF @Debug_PrintQuery = 1 PRINT @StringToExecute; EXEC sp_executesql @StringToExecute, N'@searchdisplayname nvarchar(100)', @SearchDisplayName; END GO /* So now when there's an error, it's easier to know where it's coming from: */ EXEC usp_SearchUsers @SearchDisplayName = 'Brent%', @ShowTotalVotes = 0; GO /* You may also need to log the dynamic SQL, and/or the parameters. Erik Darling wrote a solution for that: https://www.brentozar.com/archive/2017/04/fun-logging-dynamic-sql/ Basically you create a table like this: */ CREATE TABLE dbo.Logger ( Id BIGINT IDENTITY(1, 1), RunHash UNIQUEIDENTIFIER, UserName NVARCHAR(128), StartTime DATETIME2, EndTime DATETIME2, RunTimeSeconds AS DATEDIFF(SECOND, StartTime, EndTime), DynamicSQL NVARCHAR(MAX) ); /* And in your stored procedures, include this code: */ DECLARE @RunHash UNIQUEIDENTIFIER = NEWID() ... /* query-building-goes-here */ INSERT dbo.Logger ( RunHash, UserName, StartTime, EndTime, DynamicSQL ) VALUES(@RunHash, SUSER_SNAME(), SYSDATETIME(), NULL, @StringToExecute) EXEC sys.sp_executesql 'Your lousy query' UPDATE dbo.Logger SET EndTime = SYSDATETIME() WHERE RunHash = @RunHash GO /* To show you just how bad it can get, I'm going to put all the logic in at once: */ CREATE OR ALTER PROC dbo.usp_SearchUsers @SearchDisplayName NVARCHAR(100) = NULL, @SearchLocation NVARCHAR(100) = NULL, @SearchReputation INT = NULL, @OrderBy NVARCHAR(100) = 'CreationDate', @ShowTotalVotes BIT = 0, @Debug_PrintQuery TINYINT = 0, @Debug_ExecuteQuery TINYINT = 1 AS BEGIN DECLARE @StringToExecute NVARCHAR(4000), @Select NVARCHAR(4000), @From NVARCHAR(4000), @Where NVARCHAR(4000), @Order NVARCHAR(4000) = N' '; DECLARE @crlf NVARCHAR(2) = NCHAR(13) + NCHAR(10); SET @Select = @crlf + N'/* usp_SearchUsers */' + @crlf + N' select U.Id, U.DisplayName, U.Location ' + @crlf; SET @From = N' from dbo.Users U ' + @crlf; SET @Where = N' where 1 = 1 ' + @crlf; IF @SearchDisplayName IS NOT NULL SET @Where = @Where + N' and DisplayName like @searchdisplayname ' + @crlf; IF @SearchLocation IS NOT NULL SET @Where = @Where + N' and Location like @searchlocation ' + @crlf; IF @SearchReputation IS NOT NULL SET @Where = @Where + N' and Reputation = @searchreputation ' + @crlf; IF @ShowTotalVotes = 1 BEGIN SET @Select = @Select + N' , VotesCast = (select sum(1) from dbo.Votes V WHERE U.Id = V.UserId) ' + @crlf; END IF @OrderBy IS NOT NULL BEGIN SET @Order = N' order by '; SET @Order = @Order + CASE WHEN @OrderBy LIKE 'CreationDate%' THEN N' U.CreationDate ' WHEN @OrderBy LIKE 'DisplayName%' THEN N' U.DisplayName ' WHEN @OrderBy LIKE 'Location%' THEN N' U.Location ' WHEN @OrderBy LIKE 'Reputation%' THEN N' U.Reputation ' ELSE N' U.Id ' /* Or whatever default ordering you want, ideally to make SQL's life easier */ END; IF @OrderBy LIKE '% DESC' /* You could also do this with a separate @OrderByDesc bit parameter if you want */ SET @Order = @Order + N' desc '; END; SET @StringToExecute = @Select + @From + @Where + COALESCE(@Order, N' '); IF @Debug_PrintQuery = 1 PRINT @StringToExecute; IF @Debug_ExecuteQuery = 1 EXEC sp_executesql @StringToExecute, N'@searchdisplayname nvarchar(100), @searchlocation nvarchar(100), @searchreputation int', @SearchDisplayName, @SearchLocation, @SearchReputation; END GO /* Let's see how it works: */ DBCC FREEPROCCACHE; GO EXEC usp_SearchUsers @SearchDisplayName = 'Brent%', @SearchLocation = 'San Diego%', @ShowTotalVotes = 0, @OrderBy = 'DisplayName', @Debug_PrintQuery = 1; GO EXEC usp_SearchUsers @SearchLocation = 'Indiana%', @SearchReputation = 2, @ShowTotalVotes = 0, @OrderBy = 'CreationDate DESC', @Debug_PrintQuery = 1; GO EXEC usp_SearchUsers @SearchDisplayName = 'Lady Gaga%', @ShowTotalVotes = 1, @OrderBy = 'Location', @Debug_PrintQuery = 1; GO sp_BlitzCache; /* This here is the point where you wish that your conference water bottle was filled with margaritas instead, and you start sobbing into your hands. Dynamic SQL done well is really hard work, but before you complain, think about a sliding control that you can tune left to right: Fast code ___________________________________________________________ Slow code goes slow goes fast EF, NHibernate Stored Procs Dynamic SQL Start with easy-to-code stuff like ORMs. Later, as you need better performance, graduate to stored procedures. Over time, as your data grows and you have to start making tradeoffs between hardware, index size, and budgets, you may have to graduate to finely tuned dynamic SQL. Now you're better equipped to start building it in a way that scales. Wanna learn even more? Check out Erland Sommarskog's epic posts: http://www.sommarskog.se/dynamic_sql.html http://www.sommarskog.se/dyn-search.html License: Creative Commons Attribution-ShareAlike 3.0 Unported (CC BY-SA 3.0) More info: https://creativecommons.org/licenses/by-sa/3.0/ You are free to: * Share - copy and redistribute the material in any medium or format * Adapt - remix, transform, and build upon the material for any purpose, even commercially Under the following terms: * Attribution - You must give appropriate credit, provide a link to the license, and indicate if changes were made. * ShareAlike - If you remix, transform, or build upon the material, you must distribute your contributions under the same license as the original. */ |
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