1. TEMPDB – INTERNALS AND USAGE

2. AGENDA Why is tempdb so special? Myths regarding table variables
Spill warnings
Logging modifications for tempdb
Caching tables

3. WHY IS TEMPDB SO SPECIAL?
Everyone can and will use it
It can be used intentionally, none intentionally and even without our knowledge
It is the also used as the SQL Server’s developers as their “junk yard”

4. WHY IS TEMPDB SO SPECIAL?
It’s logging and table creation mechanism differ from other database’s
Whenever the servers goes down, all data in tempdb is lost

5. TEMPORARY TABLE VS. TABLE VARIABLE
In the procedures and batches we can use 2 types of objects:
Temporary table
Table variable
Both objects are created in tempdb as tables

6. MYTHS REGARDING TABLE VARIABLES
For very few records table variable will always perform better then temporary table
Operations on table variable don’t get logged
Table variable’s data is written to memory only and temporary table’s data is written to disk only

7. DEMO 1
Show that temporary tables and table variables are created as user tables in tempdb
Checking all the myths that were discussed

8. TEMPORARY TABLE VS. TABLE VARIABLE
Which type of object should we use?
For me the default choice is temporary table. I’ll use table variable only in very rare cases

9. USING TEMPDB NOT INTENTIONALLY
Sometimes the server uses the tempdb for executing a select query.
This happens mainly because of 2 reasons:
The server doesn’t have enough memory
The server misevaluates the needed memory to run a query.

10. TEMPDB AND SPILL WARNING
Such state is called spill
Unfortunately spill warnings are very hard to detect

11. DEMO 2 Creating spill with a select query
Check how to find out about spill

12. TEMPDB AND LOGGING Like every database, tempdb also uses logging
Logging is needed for:
Rollbacks
Recovery

13. TEMPDB AND LOGGING TempDB should handle rollback
TempDB should not handle recovery nor roll-forward
Because TempDB has different needs from logging, it has different logging from other databases:
Many times Inserts and updates on tempdb log only the “before state”
Many times the “logging” is done in memory only

14. TEMPDB AND LOGGING Checkpoint doesn’t write tempdb’s data to disk.
If a DBA runs the command checkpoint, it will write tempdb’s data to the disk.
Lazywriter does write tempdb’s data to disk.

15. DEMO 3
TempDB and logging

16. CACHING TABLES
Whenever the code has local temporary table or table variable, a table is created in tempdb
Creating a table is a big task
System tables have to be updated regarding the table itself, columns, indexes and constraints
The table has to get pages allocated to it

17. CACHING TABLES TempDB has a mechanism to cache tables
The server caches:
some of the system table entries
2 pages and IAM page and data page

18. CACHING TABLES Limitations of tables caching
There is no support for batches. Table creation has to be in procedure, function or trigger
You can not modify the table after its creation
You can not give the table’s constraints name

19. DEMO 4
CACHING TABLES

20. SUMERY
The myths regarding table variables being always in memory, not logged and outperform temporary tables if there are “few records” are not true.
Be aware of spill errors
When working with temporary tables, avoid modifying them after the table’s creation.

21. Thank You For Listening