SQLServer tips

This page would be a growing list for things related to SQLServer. Mostly kept for the rainy season :)

1. How to find most expensive queries (for cache query plans only) 

(ref: http://blog.sqlauthority.com/2010/05/14/sql-server-find-most-expensive-queries-using-dmv/)

SELECT TOP 10 SUBSTRING(qt.TEXT, (qs.statement_start_offset/2)+1,

((CASE qs.statement_end_offset

WHEN -1 THEN DATALENGTH(qt.TEXT)

ELSE qs.statement_end_offset

END - qs.statement_start_offset)/2)+1),

qs.execution_count,

qs.total_logical_reads, qs.last_logical_reads,

qs.total_logical_writes, qs.last_logical_writes,

qs.total_worker_time,

qs.last_worker_time,

qs.total_elapsed_time/1000000 total_elapsed_time_in_S,

qs.last_elapsed_time/1000000 last_elapsed_time_in_S,

qs.last_execution_time,

qp.query_plan

FROM sys.dm_exec_query_stats qs

CROSS APPLY sys.dm_exec_sql_text(qs.sql_handle) qt

CROSS APPLY sys.dm_exec_query_plan(qs.plan_handle) qp

ORDER BY qs.total_logical_reads DESC -- logical reads

-- ORDER BY qs.total_logical_writes DESC -- logical writes

-- ORDER BY qs.total_worker_time DESC -- CPU time

2. Queries taking longest elapsed time

SELECT TOP 10

qs.total_elapsed_time / qs.execution_count / 1000000.0 AS average_seconds,

qs.total_elapsed_time / 1000000.0 AS total_seconds,

qs.execution_count,

SUBSTRING (qt.text,qs.statement_start_offset/2,

(CASE WHEN qs.statement_end_offset = -1

THEN LEN(CONVERT(NVARCHAR(MAX), qt.text)) * 2

ELSE qs.statement_end_offset END - qs.statement_start_offset)/2) AS individual_query,

o.name AS object_name,

DB_NAME(qt.dbid) AS database_name

FROM sys.dm_exec_query_stats qs

CROSS APPLY sys.dm_exec_sql_text(qs.sql_handle) as qt

LEFT OUTER JOIN sys.objects o ON qt.objectid = o.object_id

where qt.dbid = DB_ID()

ORDER BY average_seconds DESC;

3. Queries doing most I/O

SELECT TOP 10

(total_logical_reads + total_logical_writes) / qs.execution_count AS average_IO,

(total_logical_reads + total_logical_writes) AS total_IO,

qs.execution_count AS execution_count,

SUBSTRING (qt.text,qs.statement_start_offset/2,

(CASE WHEN qs.statement_end_offset = -1

THEN LEN(CONVERT(NVARCHAR(MAX), qt.text)) * 2

ELSE qs.statement_end_offset END – qs.statement_start_offset)/2) AS individual_query,

o.name AS object_name,

DB_NAME(qt.dbid) AS database_name

FROM sys.dm_exec_query_stats qs

CROSS APPLY sys.dm_exec_sql_text(qs.sql_handle) as qt

LEFT OUTER JOIN sys.objects o ON qt.objectid = o.object_id

where qt.dbid = DB_ID()

ORDER BY average_IO DESC;

4. Find blocking and blocked queries (sessions)

Use sp_who2

Create swap file for your VPS

Just got a VPS for your own use? RAM only 512MB? or maybe you are accustomed to out of memory error and need a way to increase your memory.
Then you got to have a swap file for your VPS, the size of which will depend on your usage.

One thing to remember, swap file is created on your storage. So don't expect it to be as fast as your RAM.

Here are the steps you can follow (works on Ubuntu 12.04):
1. Check for existing swap file, if you already have one then all are good.
    swapon -s

2. Create your file for the swap file
sudo dd if=/dev/zero of=/swapfile bs=1024 count=256k

3. Set it as swap file
sudo mkswap /swapfile

4. Register the swap file
sudo swapon /swapfile

5. Make it permanent on the system
sudo nano /etc/fstab

And add the following line at the end:
/swapfile       none    swap    sw      0       0 

6. Set the swappiness
echo 10 | sudo tee /proc/sys/vm/swappiness
echo vm.swappiness = 10 | sudo tee -a /etc/sysctl.conf

7. Protect the file

sudo chown root:root /swapfile 

sudo chmod 0600 /swapfile

That's all, enjoy your VPS.

Reference:

https://www.digitalocean.com/community/tutorials/how-to-add-swap-on-ubuntu-12-04