how to improve SQL query performance in my case

Question

Hello everyone,

I have a table, schema is very simple, an ID column as unique primary key (uniqueidentifier type) and some other nvarchar columns. My current goal is, for 5000 inputs, I need to calculate what ones are already contained in the table and what are not. Tht inputs are string and I have a C# function which converts string into uniqueidentifier (GUID). My logic is, if there is an existing ID, then I treat the string as already contained in the table.

My question is, if I need to find out what ones from the 5000 input strings are already contained in DB, and what are not, what is the most efficient way?

BTW: My current implementation is, convert string to GUID using C# code, then invoke/implement a store procedure which query whether an ID exists in database and returns back to C# code.

My working environment: VSTS 2008 + SQL Server 2008 + C# 3.5.

thanks in advance, George

Answer 1

Try to ensure you end up running only one query - i.e. if your solution consists of running 5000 queries against the database, that'll probably be the biggest consumer of resources for the operation.

If you can insert the 5000 IDs into a temporary table, you could then write a single query to find the ones that don't exist in the database.

Answer 2

My first instinct would be to pump your 5000 inputs into a single-column temporary table X, possibly index it, and then use:

SELECT X.thecol
FROM X
JOIN ExistingTable USING (thecol)

to get the ones that are present, and (if both sets are needed)

SELECT X.thecol
FROM X
LEFT JOIN ExistingTable USING (thecol)
WHERE ExistingTable.thecol IS NULL

to get the ones that are absent. Worth benchmarking, at least.

Edit: as requested, here are some good docs & tutorials on temp tables in SQL Server. Bill Graziano has a simple intro covering temp tables, table variables, and global temp tables. Randy Dyess and SQL Master discuss performance issue for and against them (but remember that if you're getting performance problems you do want to benchmark alternatives, not just go on theoretical considerations!-).

MSDN has articles on tempdb (where temp tables are kept) and optimizing its performance.

Answer 3

What do you need to do with those entries that do or don't exist in your table??

Depending on what you need, maybe the new MERGE statement in SQL Server 2008 could fit your bill - update what's already there, insert new stuff, all wrapped neatly into a single SQL statement. Check it out!

• http://blogs.conchango.com/davidportas/archive/2007/11/14/SQL-Server-2008-MERGE.aspx
• http://www.sql-server-performance.com/articles/dba/SQL_Server_2008_MERGE_Statement_p1.aspx
• http://blogs.msdn.com/brunoterkaly/archive/2008/11/12/sql-server-2008-merge-capability.aspx

Your statement would look something like this:

MERGE INTO 
    (your target table) AS t
USING 
    (your source table, e.g. a temporary table) AS s
ON t.ID = s.ID
WHEN NOT MATCHED THEN  -- new rows does not exist in base table
  ....(do whatever you need to do)
WHEN MATCHED THEN      -- row exists in base table
  ... (do whatever else you need to do)
;

To make this really fast, I would load the "new" records from e.g. a TXT or CSV file into a temporary table in SQL server using BULK INSERT:

BULK INSERT YourTemporaryTable
FROM 'c:\temp\yourimportfile.csv'
WITH 
(
    FIELDTERMINATOR =',',
    ROWTERMINATOR =' |\n'
)

BULK INSERT combined with MERGE should give you the best performance you can get on this planet :-)

Marc

PS: here's a note from TechNet on MERGE performance and why it's faster than individual statements:

In SQL Server 2008, you can perform multiple data manipulation language (DML) operations in a single statement by using the MERGE statement. For example, you may need to synchronize two tables by inserting, updating, or deleting rows in one table based on differences found in the other table. Typically, this is done by executing a stored procedure or batch that contains individual INSERT, UPDATE, and DELETE statements. However, this means that the data in both the source and target tables are evaluated and processed multiple times; at least once for each statement. By using the MERGE statement, you can replace the individual DML statements with a single statement. This can improve query performance because the operations are performed within a single statement, therefore, minimizing the number of times the data in the source and target tables are processed. However, performance gains depend on having correct indexes, joins, and other considerations in place. This topic provides best practice recommendations to help you achieve optimal performance when using the MERGE statement.

Answer 4

Step 1. Make sure you have a problem to solve. Five thousand inserts isn't a lot to insert one at a time in a lot of contexts.

Are you certain that the simplest way possible isn't sufficient? What performance issues have you measured so far?

Answer 5

If you want simplicity, since 5000 records is not very many, then from C# just use a loop to generate an insert statement for each of the strings you want to add to the table. Wrap the insert in a TRY CATCH block. Send em all up to the server in one shot like this:

BEGIN TRY
INSERT INTO table (theCol, field2, field3)
SELECT theGuid, value2, value3
END TRY BEGIN CATCH END CATCH

BEGIN TRY
INSERT INTO table (theCol, field2, field3)
SELECT theGuid, value2, value3
END TRY BEGIN CATCH END CATCH

BEGIN TRY
INSERT INTO table (theCol, field2, field3)
SELECT theGuid, value2, value3
END TRY BEGIN CATCH END CATCH

if you have a unique index or primary key defined on your string GUID, then the duplicate inserts will fail. Checking ahead of time to see if the record does not exist just duplicates work that SQL is going to do anyway.

If performance is really important, then consider downloading the 5000 GUIDS to your local station and doing all the analysis localy. Reading 5000 GUIDS should take much less than 1 second. This is simpler than bulk importing to a temp table (which is the only way you will get performance from a temp table) and doing an update using a join to the temp table.

Answer 6

Definitely do not do it one-by-one.

My preferred solution is to create a stored procedure with one parameter that can take and XML in the following format:

<ROOT>
  <MyObject ID="60EAD98F-8A6C-4C22-AF75-000000000000">
  <MyObject ID="60EAD98F-8A6C-4C22-AF75-000000000001">
  ....
</ROOT>

Then in the procedure with the argument of type NCHAR(MAX) you convert it to XML, after what you use it as a table with single column (lets call it @FilterTable). The store procedure looks like:

CREATE PROCEDURE dbo.sp_MultipleParams(@FilterXML NVARCHAR(MAX))
AS BEGIN
    SET NOCOUNT ON

    DECLARE @x XML
    SELECT @x = CONVERT(XML, @FilterXML)

    -- temporary table (must have it, because cannot join on XML statement)
    DECLARE @FilterTable TABLE (
         "ID" UNIQUEIDENTIFIER
    )

    -- insert into temporary table
    -- @important: XML iS CaSe-SenSiTiv
    INSERT      @FilterTable
    SELECT      x.value('@ID', 'UNIQUEIDENTIFIER')
    FROM        @x.nodes('/ROOT/MyObject') AS R(x)

    SELECT      o.ID,
                SIGN(SUM(CASE WHEN t.ID IS NULL THEN 0 ELSE 1 END)) AS FoundInDB
    FROM        @FilterTable o
    LEFT JOIN   dbo.MyTable t
            ON  o.ID = t.ID
    GROUP BY    o.ID

END
GO

You run it as:

EXEC sp_MultipleParams '<ROOT><MyObject ID="60EAD98F-8A6C-4C22-AF75-000000000000"/><MyObject ID="60EAD98F-8A6C-4C22-AF75-000000000002"/></ROOT>'

And your results look like:

ID                                   FoundInDB
------------------------------------ -----------
60EAD98F-8A6C-4C22-AF75-000000000000 1
60EAD98F-8A6C-4C22-AF75-000000000002 0

Answer 7

Since you are using Sql server 2008, you could use Table-valued parameters. It's a way to provide a table as a parameter to a stored procedure.

Using ADO.NET you could easily pre-populate a DataTable and pass it as a SqlParameter. Steps you need to perform:

Create a custom Sql Type

CREATE TYPE MyType AS TABLE
(
UniqueId INT NOT NULL,
Column NVARCHAR(255) NOT NULL
)

Create a stored procedure which accepts the Type

CREATE PROCEDURE spInsertMyType
@Data MyType READONLY
AS 
xxxx

Call using C#

SqlCommand insertCommand = new SqlCommand(
   "spInsertMyType", connection);
 insertCommand.CommandType = CommandType.StoredProcedure;
 SqlParameter tvpParam = 
    insertCommand.Parameters.AddWithValue(
    "@Data", dataReader);
 tvpParam.SqlDbType = SqlDbType.Structured;

Links: Table-valued Parameters in Sql 2008