Excellent article once again Jeff 🙂

Nicely written and excellent Resource zip.

I have made a revision and I would be interested if you could take a look at it and let me know what you think?

It replaces the recursive CTE with my favourite toy - the Identity Hack.

On my test machine I see some speed improvement in the generation of the Hierarchy table, which starts to become noticeable around the 200,000 Employees mark and really seems to make a big improvement around the 1,000,000 employees mark (~20% overall reduction in elapsed time).

It is a modified version of your SP, so includes all your comments, plus a few of mine to indicate where and why I modified the code.

SET ANSI_NULLS ON

GO

SET QUOTED_IDENTIFIER ON

GO

CREATE PROCEDURE [dbo].[RebuildNestedSets_MM] AS

/****************************************************************************

Purpose:

Rebuilds a "Hierarchy" table that contains the original Adjacency List,

the Nested Sets version of the same hierarchy, and several other useful

columns of data some of which need not be included in the final table.

Usage:

EXEC dbo.RebuildNestedSets_MM

Progammer's Notes:

1. As currently written, the code reads from a table called dbo.Employee.

2. The Employee table must contain well indexed EmployeeID (child) and

ManagerID (parent) columns.

3. The Employee table must be a "well formed" Adjacency List. That is, the

EmployeeID column must be unique and there must be a foreign key on the

ManagerID column that points to the EmployeeID column. The table must not

contain any "cycles" (an EmployeeID in its own upline). The Root Node

must have a NULL for ManagerID.

4. The final table, named dbo.Hierarchy, will be created in the same

database as where this stored procedure is present. IT DOES DROP THE

TABLE CALLED DBO.HIERARCHY SO BE CAREFUL THAT IT DOESN'T DROP A TABLE

NEAR AND DEAR TO YOUR HEART.

5. This code currently has no ROLLBACK capabilities so make sure that you

have met all of the requirements (and, perhaps, more) cited in #3 above.

Dependencies:

1. This stored procedure requires that the following special purpose HTally

table be present in the same database from which it runs.

--===== Create the HTally table to be used for splitting SortPath

SELECT TOP 1000 --(4 * 1000 = VARBINARY(4000) in length)

N = ISNULL(CAST(

(ROW_NUMBER() OVER (ORDER BY (SELECT NULL))-1)*4+1

AS INT),0)

INTO dbo.HTally

FROM master.sys.all_columns ac1

CROSS JOIN master.sys.all_columns ac2

;

--===== Add the quintessential PK for performance.

ALTER TABLE dbo.HTally

ADD CONSTRAINT PK_HTally

PRIMARY KEY CLUSTERED (N) WITH FILLFACTOR = 100

;

Revision History:

Rev 00 - Circa 2009 - Jeff Moden

- Initial concept and creation.

Rev 01 - PASS 2010 - Jeff Moden

- Rewritten for presentation at PASS 2010.

Rev 02 - 06 Oct 2012 - Jeff Moden

- Code redacted to include a more efficient, higher performmance

method of splitting the SortPath using a custom HTally Table.

Sug 00 - 13 NOv 2012 - Mister Magoo

- CTE replaced with Magoo's Identity Hack for a more efficient recursion

- Also removes the need to tweak MAXRECURSION

****************************************************************************/

--===========================================================================

-- Presets

--===========================================================================

--===== Suppress the auto-display of rowcounts to prevent from returning

-- false errors if called from a GUI or other application.

SET NOCOUNT ON;

--===== Start a duration timer

DECLARE @StartTime DATETIME,

@Duration CHAR(12);

SELECT @StartTime = GETDATE();

--===========================================================================

-- 1. Read ALL the nodes in a given level as indicated by the parent/

-- child relationship in the Adjacency List.

-- 2. As we read the nodes in a given level, mark each node with the

-- current level number.

-- 3. As we read the nodes in a given level, convert the EmployeeID to

-- a Binary(4) and concatenate it with the parents in the previous

-- level's binary string of EmployeeID's. This will build the

-- SortPath.

-- 4. Number the rows according to the Sort Path. This will number the

-- rows in the same order that the push-stack method would number

-- them.

--===========================================================================

--===== Conditionally drop the final table to make reruns easier in SSMS.

IF OBJECT_ID('FK_Hierarchy_Hierarchy') IS NOT NULL

ALTER TABLE dbo.Hierarchy

DROP CONSTRAINT FK_Hierarchy_Hierarchy;

IF OBJECT_ID('dbo.Hierarchy','U') IS NOT NULL

DROP TABLE dbo.Hierarchy;

RAISERROR('Building the initial table and SortPath...',0,1) WITH NOWAIT;

--===== Mister Magoo Mods Start

--===== Build the tmep table for the Identity Hack

CREATE TABLE #BuildPath(

EmployeeIDINT NOT NULL ,

ManagerIDINT,

HLevelSMALLINT IDENTITY(1,1) NOT NULL,

LeftBowerINT NOT NULL,

RightBowerINT NOT NULL,

NodeNumberINT NOT NULL,

NodeCountINT NOT NULL,

SortPathVARBINARY(4000)

);

--===== Add a nice index to help with selecting the levels for recursion.

CREATE CLUSTERED INDEX ix_cl ON #BuildPath(HLevel);

--===== Turn on identity insert so that we can put our own values into the HLevel column.

SET IDENTITY_INSERT #BuildPath ON;

--===== Insert the top level Manager(s) into the temp table

INSERT #BuildPath(

EmployeeID ,

ManagerID ,

HLevel ,

LeftBower,

RightBower,

NodeNumber,

NodeCount,

SortPath )

SELECT anchor.EmployeeID,

anchor.ManagerID,

HLevel = 1,

LeftBower = 0,

RightBower = 0,

NodeNumber = 0,

NodeCount = 0,

SortPath = CAST(

CAST(anchor.EmployeeID AS BINARY(4))

AS VARBINARY(4000)) --Up to 1000 levels deep.

FROM dbo.Employee AS anchor

WHERE ManagerID IS NULL; --Only the Root Node has a NULL ManagerID

--==== This is the "recursive" part of the Identity Hack that adds 1 for each level

-- and concatenates each level of EmployeeID's to the SortPath column.

-- It uses SCOPE_IDENTITY to "read" the last level processed.

WHILE @@ROWCOUNT>0

INSERT #BuildPath(

EmployeeID ,

ManagerID ,

HLevel ,

LeftBower,

RightBower,

NodeNumber,

NodeCount,

SortPath )

SELECT recur.EmployeeID,

recur.ManagerID,

HLevel = SCOPE_IDENTITY() + 1,

LeftBower = 0,

RightBower = 0,

NodeNumber = 0,

NodeCount = 0,

SortPath = CAST( --This does the concatenation to build SortPath

cte.SortPath + CAST(Recur.EmployeeID AS BINARY(4))

AS VARBINARY(4000))

FROM dbo.Employee AS recur WITH (TABLOCK)

JOIN #BuildPath as cte WITH(TABLOCK)

ON cte.EmployeeID = recur.ManagerID

WHERE cte.HLevel=SCOPE_IDENTITY();

SET IDENTITY_INSERT #BuildPath OFF;

--=== This final INSERT/SELECT creates the Node # in the same order as a

-- push-stack would. It also creates the final table with some

-- "reserved" columns on the fly. We'll leave the SortPath column in

-- place because we're still going to need it later.

-- The ISNULLs make NOT NULL columns

SELECT EmployeeID = sorted.EmployeeID,

sorted.ManagerID,

HLevel = sorted.HLevel,

LeftBower , --Place holder

RightBower , --Place holder

NodeNumber = ROW_NUMBER() OVER (ORDER BY sorted.SortPath),

NodeCount , --Place holder

SortPath = sorted.SortPath

INTO dbo.Hierarchy

FROM #BuildPath AS sorted;

RAISERROR('There are %u rows in dbo.Hierarchy',0,1,@@ROWCOUNT) WITH NOWAIT;

--===== Mister Magoo Mods End

--===== Display the cumulative duration

SELECT @Duration = CONVERT(CHAR(12),GETDATE()-@StartTime,114);

RAISERROR('Cumulative Duration = %s',0,1,@Duration) WITH NOWAIT;

--===========================================================================

-- Using the information created in the table above, create the

-- NodeCount column and the LeftBower and RightBower columns to create

-- the Nested Sets hierarchical structure.

--===========================================================================

RAISERROR('Building the Nested Sets...',0,1) WITH NOWAIT;

--===== Declare a working variable to hold the result of the calculation

-- of the LeftBower so that it may be easily used to create the

-- RightBower in a single scan of the final table.

DECLARE @LeftBower INT

;

--===== Create the Nested Sets from the information available in the table

-- and in the following CTE. This uses the proprietary form of UPDATE

-- available in SQL Serrver for extra performance.

WITH cteCountDownlines AS

( --=== Count each occurance of EmployeeID in the sort path

SELECT EmployeeID = CAST(SUBSTRING(h.SortPath,t.N,4) AS INT),

NodeCount = COUNT(*) --Includes current node

FROM dbo.Hierarchy h,

dbo.HTally t

WHERE t.N BETWEEN 1 AND DATALENGTH(SortPath)

GROUP BY SUBSTRING(h.SortPath,t.N,4)

) --=== Update the NodeCount and calculate both Bowers

UPDATE h

SET @LeftBower = LeftBower = 2 * NodeNumber - HLevel,

h.NodeCount = downline.NodeCount,

h.RightBower = (downline.NodeCount - 1) * 2 + @LeftBower + 1

FROM dbo.Hierarchy h

JOIN cteCountDownlines downline

ON h.EmployeeID = downline.EmployeeID

;

RAISERROR('%u rows have been updated to Nested Sets',0,1,@@ROWCOUNT)

WITH NOWAIT;

RAISERROR('If the rowcounts don''t match, there may be orphans.'

,0,1,@@ROWCOUNT)WITH NOWAIT;

--===== Display the cumulative duration

SELECT @Duration = CONVERT(CHAR(12),GETDATE()-@StartTime,114);

RAISERROR('Cumulative Duration = %s',0,1,@Duration) WITH NOWAIT;

--===========================================================================

-- Prepare the table for high performance reads by adding indexes.

--===========================================================================

RAISERROR('Building the indexes...',0,1) WITH NOWAIT;

--===== Direct support for the Nested Sets

ALTER TABLE dbo.Hierarchy

ADD CONSTRAINT PK_Hierarchy

PRIMARY KEY CLUSTERED (LeftBower, RightBower) WITH FILLFACTOR = 100

;

CREATE UNIQUE INDEX AK_Hierarchy

ON dbo.Hierarchy (EmployeeID) WITH FILLFACTOR = 100

;

ALTER TABLE dbo.Hierarchy

ADD CONSTRAINT FK_Hierarchy_Hierarchy FOREIGN KEY

(ManagerID) REFERENCES dbo.Hierarchy (EmployeeID)

ON UPDATE NO ACTION

ON DELETE NO ACTION

;

--===== Display the cumulative duration

SELECT @Duration = CONVERT(CHAR(12),GETDATE()-@StartTime,114);

RAISERROR('Cumulative Duration = %s',0,1,@Duration) WITH NOWAIT;

--===========================================================================

-- Exit

--===========================================================================

RAISERROR('===============================================',0,1) WITH NOWAIT;

RAISERROR('RUN COMPLETE',0,1) WITH NOWAIT;

RAISERROR('===============================================',0,1) WITH NOWAIT;