SQL script executes forever - possible loop? - sql

Question

I hope that someone can help me with this script.
I wanted to create an SQL script to rebuild all non clustered index with "online ON" option to prevent locking the tables while rebuilding the indexes, and also without using a cursor (which slow things down). For that reason I used While loop (at least what most SQL developers suggested), so I wrote an SQL script.
What I noticed, SQL script executes forever. On one test machine I left it running for 12 minutes, after which I aborted the operation...
Can some SQL guru please tell me what is causing a bottle neck?
Is there a better way to do this, or perhaps to "fine tune" current script?
Sample of old code:
USE MASTER
GO
DECLARE #DbName AS VARCHAR(50),
#DBIndexName AS VARCHAR(250),
#DBTableName AS VARCHAR(100),
#AlterCommand AS VARCHAR(500),
#SwitchDB AS NVARCHAR(50)
SELECT name INTO #DatabaseList FROM master..sysdatabases
WHILE EXISTS (SELECT * FROM #DatabaseList)
BEGIN
SELECT TOP 1 #DbName = name FROM #DatabaseList
ORDER BY name ASC
SET #SwitchDB = 'USE '+#DbName
EXEC(#SwitchDB)
-- pronaći sve index-e koji imaju fragmentaciju veću od 10%, te staviti u tablicu
SELECT object_name(dt.object_id) Tablename,si.name
IndexName,dt.avg_fragmentation_in_percent AS
ExternalFragmentation,dt.avg_page_space_used_in_percent AS
InternalFragmentation
INTO #FragmIndex
FROM
(
SELECT object_id,index_id,avg_fragmentation_in_percent,avg_page_space_used_in_percent
FROM sys.dm_db_index_physical_stats (db_id(#DbName),null,null,null,'DETAILED')
WHERE index_id <> 0
) AS dt INNER JOIN sys.indexes si ON si.object_id=dt.object_id
AND si.index_id=dt.index_id AND dt.avg_fragmentation_in_percent>10
AND dt.avg_page_space_used_in_percent<75 and si.type_desc = 'NONCLUSTERED' ORDER BY avg_fragmentation_in_percent DESC
-- Napraviti petlju koja će izvršiti rebuild svih indexa
SET #SwitchDB = 'USE master'
EXEC (#SwitchDB)
WHILE EXISTS (SELECT * FROM #FragmIndex)
BEGIN
SELECT TOP 1 #DBIndexName = IndexName, #DBTableName = Tablename FROM #FragmIndex
ORDER BY IndexName ASC
-- rebuild index command setiranje na "ONLINE ON" kako bi se izbjeglo "zaključavanje tablice".
SET #AlterCommand ='ALTER INDEX '+#DBIndexName+' ON '+ #DbName+'.dbo.'+#DbTableName +'REBUILD WITH (FILLFACTOR=80,STATISTICS_NORECOMPUTE = ON,ONLINE=ON)'
EXEC(#AlterCommand)
DELETE #FragmIndex
WHERE IndexName = #DBIndexName
END
DELETE #DatabaseList
WHERE name = #DbName
END
DROP TABLE #DatabaseList
GO
Thank you very much, and best regards.
Just to add new "corrected code" - for all of you that wish to use it, modify it or whatever... :)
USE MASTER
GO
DECLARE #DbName AS VARCHAR(50),
#DBIndexName AS VARCHAR(250),
#DBTableName AS VARCHAR(100),
#AlterCommand AS VARCHAR(500),
#SwitchDB AS NVARCHAR(50),
#numEntries AS BIGINT,
#numIndexEntries AS BIGINT
SELECT name INTO #DatabaseList FROM master..sysdatabases
SET #numEntries = (SELECT COUNT(*) FROM #DatabaseList)
WHILE #numEntries > 0 --EXISTS (SELECT * FROM #DatabaseList)
BEGIN
SELECT TOP 1 #DbName = name FROM #DatabaseList
ORDER BY name ASC
SET #SwitchDB = 'USE '+#DbName
EXEC(#SwitchDB)
-- pronaći sve index-e koji imaju fragmentaciju veću od 10%, te staviti u tablicu
SELECT object_name(dt.object_id) Tablename,si.name
IndexName,dt.avg_fragmentation_in_percent AS
ExternalFragmentation,dt.avg_page_space_used_in_percent AS
InternalFragmentation
INTO #FragmIndex
FROM
(
SELECT object_id,index_id,avg_fragmentation_in_percent,avg_page_space_used_in_percent
FROM sys.dm_db_index_physical_stats (db_id(#DbName),null,null,null,'DETAILED')
WHERE index_id <> 0
) AS dt INNER JOIN sys.indexes si ON si.object_id=dt.object_id
AND si.index_id=dt.index_id AND dt.avg_fragmentation_in_percent>10
AND dt.avg_page_space_used_in_percent<75 and si.type_desc = 'NONCLUSTERED' ORDER BY avg_fragmentation_in_percent DESC
-- Napraviti petlju koja će izvršiti rebuild svih indexa
SET #SwitchDB = 'USE master'
EXEC (#SwitchDB)
SET #numIndexEntries = (SELECT COUNT(*) FROM #FragmIndex)
WHILE #numIndexEntries > 0 --EXISTS (SELECT * FROM #FragmIndex)
BEGIN
SELECT TOP 1 #DBIndexName = IndexName, #DBTableName = Tablename FROM #FragmIndex
ORDER BY IndexName ASC
-- rebuild index command setiranje na "ONLINE ON" kako bi se izbjeglo "zaključavanje tablice".
SET #AlterCommand ='ALTER INDEX '+#DBIndexName+' ON '+ #DbName+'.dbo.'+#DbTableName +'REBUILD WITH (FILLFACTOR=80,STATISTICS_NORECOMPUTE = ON,ONLINE=ON)'
EXEC(#AlterCommand)
DELETE #FragmIndex WHERE IndexName = #DBIndexName
SET #numIndexEntries = (SELECT COUNT(*) FROM #FragmIndex)
END
DELETE #DatabaseList WHERE name = #DbName
SET #numEntries = (SELECT COUNT(*) FROM #DatabaseList)
END
DROP TABLE #DatabaseList
GO
P.S: if you have any other suggestions, or found bug in the script - for improving the code, please let me know.

Answer 1

Two possible problems might cause that effect.
First of all, the DELETE statement may not really delete anything. You can easily check this by PRINTing the number of entries in #DatabaseList after every DELETE statement.
Secondly, the EXISTS (SELECT * FROM #DatabaseList) could just be evaluated once, not upon every iteration of the loop.
I'd change this as follows:
...
DECLARE #numEntries BIGINT
SET #numEntries = (SELECT COUNT(*) FROM #DatabaseList)
WHILE #numEntries > 0
BEGIN
...
DELETE #DatabaseList WHERE name = #DbName
SET #numEntries = (SELECT COUNT(*) FROM #DatabaseList)
END
...