1. “Magic numbers”, local variable and performance
Як стиль написання запитів впливає на швидкодію системи
Андрій Зробок

2. Need Java / NET programmer
Java/NET programming – 80%
SQL programming – 20%
Framework
Query builder
Simple queries
etc.
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4/7/2019 |
Magic numbers, local variables and performance

3. Java “Magic numbers” anti-pattern
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Magic numbers, local variables and performance

4. Test data database: AdventureWorks2012 table: Person.Address 4 |
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Magic numbers, local variables and performance

5. “Magic numbers” in SQL –code
/*original code*/
select addressid, AddressLine1, addressline2, city
from [Person].[Address] where [StateProvinceID] = 9 go
from [Person].[Address] where [StateProvinceID] = 119
/*refactored code*/
int = 9
select addressid, AddressLine1, addressline2, city
from [Person].[Address] where [StateProvinceID] go
int = 119
from [Person].[Address] where [StateProvinceID]
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Magic numbers, local variables and performance

6. Original queries (IO stat, execution plans)
(4564 row(s) affected)
Table 'Address'.
Scan count 1,
logical reads 216, …
(1 row(s) affected)
logical reads 4, …
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Magic numbers, local variables and performance

7. Original queries (Rows Estimated vs Actual)
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Magic numbers, local variables and performance

8. “New” queries (IO stat, execution plans)
(4564 row(s) affected)
Table 'Address'.
Scan count 1,
logical reads 216, …
(1 row(s) affected)
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Magic numbers, local variables and performance

9. “New” queries (Rows Estimated vs Actual)
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Magic numbers, local variables and performance

10. Statistics.
DBCC SHOW_STATISTICS ('[Person].[Address]', IX_Address_StateProvinceID)
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Magic numbers, local variables and performance

11. Statistics: Estimated Row Count
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Magic numbers, local variables and performance

12. Statistics: 30% declare @i int = 179
select addressid, AddressLine1, addressline2, city
from [Person].[Address] where [StateProvinceID] go
(62 row(s) affected)
Table 'Address'. Scan count 1,
logical reads 216, …
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Magic numbers, local variables and performance

13. Statistics: 30%
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Magic numbers, local variables and performance

14. Statistics: 30% declare @i int = 0
select addressid, AddressLine1, addressline2, city
from [Person].[Address] where [StateProvinceID] go
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Magic numbers, local variables and performance

15. Execution plan – re-usage
Parser – makes sure that the T-SQL query has a
valid syntax,
Output – Parse Tree
Algebrizer – is responsible for objects and
columns names verification. identifies all data types
which are being processed for a given query. verify
that GROUP BY and aggregate columns are placed
in right place or not
Output – Algebrized Tree
Optimizer – execution plan creation
Output – Execution Plan
Execution – query execution
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Magic numbers, local variables and performance

16. Execution plan – re-usage
SELECT A.col5, SUM(C.col6) AS col6sum
FROM TableA AS A
INNER JOIN TableB AS B ON A.col1 = B.col1
INNER JOIN TableC AS C ON B.col2 = c.col2
WHERE A.col3 = constant1
AND B.col4 = constant2
GROUP BY A.col5;
FROM (3! = 6 OR (4! / 2!) = 12 variants) :
left-deep tree: JOIN( JOIN( JOIN(A, B), C), D). n! (6 tables – 720)
bushy tree: JOIN(JOIN(A, B), JOIN(C, D)). (2n-2)!/(n-1)! (6 tables – 30,240)
Realization (connection type): (Hash, merge, nested loop, star): 4
Access (index seek, index scan, table scan): 3
WHERE: AND : 2
GROUP BY: 2 ways (Stream (ordered) / Hash Match aggregate )
6*4*2*2*3 = 288 OR 12 *4*2*2*3 = 576
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Magic numbers, local variables and performance

17. Parameterization auto forced manual DBCC FREEPROCCACHE GO
select addressid, AddressLine1, addressline2, city
from [Person].[Address] where [StateProvinceID] = 9
int = 119
from [Person].[Address]
where [StateProvinceID]
from [Person].[Address] where [StateProvinceID] = 119
int = 9
where [StateProvinceID]
auto
forced
manual
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Magic numbers, local variables and performance

18. Parameterization: Adhoc queries
SELECT usecounts, cacheobjtype, objtype, text
FROM sys.dm_exec_cached_plans
CROSS APPLY sys.dm_exec_sql_text(plan_handle)
WHERE text like '%Address%'
ORDER BY usecounts DESC; GO
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Magic numbers, local variables and performance

19. Auto Parameterization
select addressid, addressline1, addressline2, city from [Person].[Address] where [AddressID] = 9; go
select addressid, addressline1, addressline2, city from [Person].[Address] where [AddressID] = 119;
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Magic numbers, local variables and performance

20. Forced Parameterization
alter database AdventureWorks2012 set parameterization forced; go
select addressid, addressline1, addressline2, city from [Person].[Address] where [StateProvinceID] = 9;
select addressid, addressline1, addressline2, city from [Person].[Address] where [StateProvinceID] = 119;
alter database AdventureWorks2012 set parameterization simple;
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Magic numbers, local variables and performance

21. Manual Parameterization (sp_executeSQL)
nvarchar(max)
nvarchar(max)
int
= N'select addressid, addressline1, addressline2, city from [Person].[Address] where [StateProvinceID]
= int'
= 9
execute
= 119 go
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Magic numbers, local variables and performance

22. Manual Parameterization (procedures)
create procedure dbo.get_address
int ) as
begin
select addressid, addressline1, addressline2, city
from [Person].[Address]
where [StateProvinceID]
end
exec dbo.get_address 9 go
exec dbo.get_address 119
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Magic numbers, local variables and performance

23. SP side effect: non-effective plan
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Magic numbers, local variables and performance

24. SP side effect: non-effective plan
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Magic numbers, local variables and performance

25. Parameterization side effect: data conversion
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Magic numbers, local variables and performance

26. Non-effective plan: how to avoid
exec dbo.get_address 9 go
exec sp_recompile 'dbo.get_address'
exec dbo.get_address 119
exec dbo.get_address 9 go
exec dbo.get_address 119 WITH RECOMPILE
create procedure dbo.get_address
int) WITH RECOMPILE as
begin
select addressid, addressline1, addressline2, city
from [Person].[Address] where [StateProvinceID]
end go
Query hint: OPTION (RECOMPILE)
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Magic numbers, local variables and performance

27. local variable: how to generate optimal plan
int = 119
select addressid, AddressLine1, addressline2, city
from [Person].[Address] where [StateProvinceID]
OPTION (recompile) go
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Magic numbers, local variables and performance

28. No caching Option (Recompile) declare @state_gironde_FR int = 119
select addressid, AddressLine1, addressline2, city
from [Person].[Address] where [StateProvinceID]
OPTION (recompile) go
select addressid, AddressLine1, addressline2, city
from [Person].[Address] where [StateProvinceID] = 119 go
No caching
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Magic numbers, local variables and performance

29. Circle closed Option (recompile) Ad hoc queries Stored procedures
Parameterization
Stored procedures
Option (recompile)
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Magic numbers, local variables and performance

30. Side effect of the parameter sniffing
create procedure dbo.get_address_sniffing
int ) as
begin
+ 1
select addressid, addressline1, addressline2, city
from [Person].[Address]
where [StateProvinceID]
end Go
DBCC FREEPROCCACHE
exec dbo.get_address_sniffing 9 go
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Magic numbers, local variables and performance

31. Side effect (parameter sniffing): how to avoid
create procedure dbo.get_address_sniffing
int ) as
begin
+ 1
select addressid, addressline1, addressline2, city
from [Person].[Address]
where [StateProvinceID]
OPTION (recompile)
end Go
DBCC FREEPROCCACHE
exec dbo.get_address_sniffing 9 go
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Magic numbers, local variables and performance

32. Option (Recompile): data filtering
if object_id('dbo.get_address_by_filter','P') is not null
drop procedure dbo.get_address_by_filter go
create procedure dbo.get_address_by_filter (
@city nvarchar(30) = null
nvarchar(15) = null
int = null ) as
begin
SELECT addressid, AddressLine1, addressline2, city
FROM [AdventureWorks2012].[Person].[Address]
where (city is null)
and (postalcode is null)
and (stateprovinceid is null)
end
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Magic numbers, local variables and performance

33. Option (Recompile): data filtering
execute dbo.get_address_by_filter @stateprovinceid = 48 = N'Las Cruces'
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Magic numbers, local variables and performance

34. Option (Recompile): data filtering
if object_id('dbo.get_address_by_filter','P') is not null
drop procedure dbo.get_address_by_filter go
create procedure dbo.get_address_by_filter (
@city nvarchar(30) = null
nvarchar(15) = null
int = null ) as
begin
SELECT addressid, AddressLine1, addressline2, city
FROM [AdventureWorks2012].[Person].[Address]
where (city is null)
and (postalcode is null)
and (stateprovinceid is null)
option (recompile)
end
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Magic numbers, local variables and performance

35. Option (Recompile): data filtering
execute dbo.get_address_by_filter @stateprovinceid = 48 = N'Las Cruces'
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Magic numbers, local variables and performance

36. Option (Recompile): data ordering
if object_id('dbo.get_address_by_order','P') is not null
drop procedure dbo.get_address_by_order go
create procedure dbo.get_address_by_order (
@orderby nvarchar(254) ) as
begin
SELECT top 100 addressid, AddressLine1, addressline2, city
FROM [AdventureWorks2012].[Person].[Address]
order by
when N'city' then city
when N'postalcode' then postalcode
when N'stateprovinceid' then stateprovinceid
end
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Magic numbers, local variables and performance

37. Option (Recompile): data ordering
execute dbo.get_address_by_order N'stateprovinceid'
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Magic numbers, local variables and performance

38. Option (Recompile): data ordering
if object_id('dbo.get_address_by_order','P') is not null
drop procedure dbo.get_address_by_order go
create procedure dbo.get_address_by_order (
@orderby nvarchar(254) ) as
begin
SELECT top 100 addressid, AddressLine1, addressline2, city
FROM [AdventureWorks2012].[Person].[Address]
order by
when N'city' then city
when N'postalcode' then postalcode
when N'stateprovinceid' then stateprovinceid
End
Option (recompile)
end
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Magic numbers, local variables and performance

39. Option (Recompile): data ordering
execute dbo.get_address_by_order N'stateprovinceid'
SELECT top 100
addressid,
AddressLine1,
addressline2,
city
FROM [Person].[Address]
order by stateprovinceid
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Magic numbers, local variables and performance

40. Option (Recompile): data ordering
execute dbo.get_address_by_order N‘city'
order by
when N'city' then city
when N'postalcode' then postalcode
when N'stateprovinceid' then stateprovinceid
End
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Magic numbers, local variables and performance

41. Option (Recompile): data ordering
if object_id('dbo.get_address_by_order','P') is not null
drop procedure dbo.get_address_by_order go
create procedure dbo.get_address_by_order (
@orderby nvarchar(254) ) as
begin
SELECT top 100 addressid, AddressLine1, addressline2, city
FROM [AdventureWorks2012].[Person].[Address]
order by
when N'city' then city
when N'postalcode' then postalcode
when N'stateprovinceid' then stateprovinceid
else cast('' as sql_variant)
end
option (recompile)
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Magic numbers, local variables and performance

42. “Magic numbers”, local variable and performance
Q & A
The end
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Magic numbers, local variables and performance