Optimizing database access

and diving into .Net SqlClient

Mladen PrajdićSQL Server MVP

mladenp@gmail.com @MladenPrajdic

About me

Welcome to SloveniaThe sunny side of Alps

Disclaimer

I use SELECT * in demos only for

brevity

Agenda• Building foundation... Client Data Access

• New exciting stuff! Async SqlDataReader

• Consistency is the key! TransactionScope

• Reduce, Reuse, Recycle! Connection Pooling

• Make it go faster! Batching Commands

• What’s it doing? Statistics

• Multiple interleaves? MARS

Correct Connection Use

using (SqlConnection conn = new SqlConnection(_connString)){ conn.Open(); // db access code here // return data reader or objects}

Client Data Access

Building the foundation...

TDS Data Flow

Result set

“Done” token

Result set

“Done” token

Result set

“Done” token

using (SqlCommand cmd = new SqlCommand("SELECT * FROM Table1; SELECT * FROM Table2", conn)){ SqlDataReader rdr = cmd.ExecuteReader(CommandBehavior.CloseConnection); // return the list of data objects return CreateListOfBusinessObjects(rdr); // or return the SqlDataReader itself return rdr;}

TDS Data Flow

Result set

“Done” token

Row Data“Row” token Row Data“Row”

token Row Data“Row” token

Result set

“Done” token

Result set

“Done” token

TDS Data Flow

Result set

“Done” token

Row Data“Row” token Row Data“Row”

token Row Data“Row” token

Result set

“Done” token

Result set

“Done” token

Column Data

Column

Header

Column DataColum

n Heade

r

Column Data

Column

Header

Small datatype INT Medium datatype CHAR(50)Large datatype NVARCHAR(MAX)

SqlDataReader Data Access• Default access (Non-Sequential)

• Sequential access

// default = non-sequential accessvar rdr = cmd.ExecuteReader();

// sequential accessvar rdr = cmd.ExecuteReader(CommandBehavior.SequentialAccess);

Non-Sequential (Default) Access

Column Data

Column

Header

Column DataColum

n Heade

r

Column Data

Column

Header

0 1 2

Column Data

Column

Header

Column DataColum

n Heade

r

Column Data

Column

Header

0 1 2

We can go back and read all previous columns’ data

Non-Sequential (Default) Access• The full row is stored in client memory• Move back and forward in the row• Possible scalability issues• Possible Out of Memory exceptions

Sequential Access

Column Data

Column

Header

Column DataColum

n Heade

r

Column Data

Column

Header

0 1 2

Column Data

Column

Header

Column DataColum

n Heade

r

Column Data

Column

Header

0 1 2

Forward reading only. Trying to go back throws an error.

ERROR

Sequential Access• Only the current value is stored in client

memory• Move only forward in the row• Get* methods for getting data• GetBytes, GetChars for BLOBs• Save to a stream by reading it in chunks// get first column value that is intrdr.GetInt32(0);// get second column value that is binarybyte[] buffer = new byte[100];// loop this to get the whole column out in chunksrdr.GetBytes(0, 100, buffer, 0, buffer.Length); // save the byte buffer to file or something

Sequential Access• Full streaming support in .Net 4.5• GetStream, GetTextReader, GetXmlReader• No more chunking

Sequential AccessSqlCommand readCmd = // get varbinary(MAX) from source table new SqlCommand("SELECT BinData FROM Source", readConn);SqlCommand writeCmd = // insert varbinary(MAX) into target table new SqlCommand("INSERT INTO Target(BinData) VALUES (@BinData)", writeConn);// Size is set to -1 to indicate "MAX"SqlParameter streamParameter = writeCmd.Parameters.Add("@BinData", SqlDbType.Binary, -1);SqlDataReader readerSource = readCmd.ExecuteReader(CommandBehavior.SequentialAccess);while (readerSource.Read()){ // Grab a stream to the binary data in the source database Stream dataStream = readerSource.GetStream(0); // Set the parameter value to the stream source that was opened streamParameter.Value = dataStream; // send the data to the target database writeCmd.ExecuteNonQuery();}

Async SqlDataReader

New exciting stuff!

Old “Run in Background” Ways

SqlCommand cmd = new SqlCommand("SELECT * FROM Table", conn);

// either this or a background workerIAsyncResult result = cmd.BeginExecuteReader();

// for the reader to completewhile (!result.IsCompleted){ // do UI operations without problems}

using (SqlDataReader reader = cmd.EndExecuteReader(result)){ // show reader results ShowDbOperationResult(reader);}

What exactly is the new stuff then?• Make C# async programming easier• Since .Net 4.5• Supports easy cancellation

• Uses the new async/await keywords• Reader’s *Async methodsprivate async void CustomEventHandler(object sender, EventArgs e){ textBox.Text = "Start long operation." + Environment.NewLine; // await returns control to the caller // this means that the UI won't be blocked // since CPU is free to do its stuff double duration = await MyLongIOBoundOperation(); // finish op and show duration in UI textBox.Text = "Completed in " + duration + " sec";}

What is it good for?• Make code async without complex threading

code• No threads created by default• Use default for IO-bound workload• Use Task.Run to create background thread for

CPU-bound workload• Supported by Entity Framework 6

• For more in depth details on this, search for “Asynchronous Programming with Async and Await”• http://

msdn.microsoft.com/en-us/library/hh191443.aspx

What to use when?• Prefer NextResultAsync in either access

modes

• TDS tokens between result sets are processed in async

Result set TDSTokens

Async

Result set TDSTokens

Async

Result set

What to use when?• Prefer ReadAsync in either access modes

• TDS tokens between multiple packets are processed in async

Data Packet TDSTokens

Async

Data Packet TDSTokens

Async

Data Packet

Single Row

What to use when?• Use IsDBNull and GetFieldValue<T> • If ReadAsync() in Default (Non-Sequential) mode

• Since data is already in the client buffer

• IsDBNullAsync and GetFieldValueAsync<T>• Sequential mode

• target column is large• needed to read past large columns

• rdr.GetStream(i).CopyToAsync(outputStream)• for huge columns in Sequential mode

DEMO

TransactionScope

Consistency is the key!

TransactionScope

using (TransactionScope txScope = new TransactionScope()){ using (SqlConnection conn2012AW = new SqlConnection(_connString2012AW)) { // do stuff } txScope.Complete();}

TransactionScope• Easy to use• Keeps code simple• Great transaction management out of the box• MSTDC escalation• Always, unless connecting to the same database

with non concurrent connections• sys.dm_tran_active_transactions.transaction_uow

• Default isolation level is SERIALIZABLE

DEMO

Connection Pooling

Reduce, Reuse, Recycle!

Connection Pooling• Connection pools created• per process• per application domain • per connection string• per Windows identity (if integrated security) 

• Default max pool size = 100• Audit Login, Audit Logout Events• EventSubClass

• 1 = Nonpooled• 2 = Pooled

Connection Pooling Resets• sp_reset_connection• Resets the connection for reuse• A long list of what it does on the next slide for

reference

• Does NOT reset the isolation level• Always explicitly specify the isolation level

Connection Pooling Resets• All error states and numbers (like @@error)• Stops all EC's (execution contexts) that are child threads of a parent EC executing a parallel query• Waits for any outstanding I/O operations that is outstanding• Frees any held buffers on the server by the connection• Unlocks any buffer resources that are used by the connection• Releases all allocated memory owned by the connection• Clears any work or temporary tables that are created by the connection• Kills all global cursors owned by the connection• Closes any open SQL-XML handles that are open• Deletes any open SQL-XML related work tables• Closes all system tables• Closes all user tables• Drops all temporary objects• Aborts open transactions• Defects from a distributed transaction when enlisted• Decrements the reference count for users in current database which releases shared database locks• Frees acquired locks• Releases any acquired handles• Resets all SET options to the default values• Resets the @@rowcount value• Resets the @@identity value• Resets any session level trace options using dbcc traceon()

DEMO

Batching Commands

Make it go faster!

Batching Commands• Optimizing network round trips• Consider Bulk Insert if possible• Streaming data insert• SqlBulkCopy Class

• 10k inserts = 10k round trips• 1000 x 10 batches = 10 round trips• 1000 CSV SQL statements in one command• Or… a hidden gem

DEMO

Statistics

What’s it doing?

Client Statistics• Enable per connection• Cumulative• Good for tracking the actual db access from

app

• Save to a queue, process off-peak• Analyze historical access data

DEMO

MARS

Multiple interleaves?

Multiple Active Result Sets (MARS)• Statement multiplexing/interleaving • SELECT, FETCH• RECEIVE, READTEXT• BULK INSERT / BCP

• Other statements must complete• Transaction savepoints aren’t allowed

• Usually used for SELECT N+1 scenarios

DEMO

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