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minimal-logging-demo.sql
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minimal-logging-demo.sql
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set statistics io on
if object_id('dbo.Test', 'U') is not null
begin
drop table dbo.Test;
end;
go
create table dbo.Test
(
id int not null identity constraint pk_test_id -- [PK dbo.Test (id)]
primary key,
c1 int not null,
padding char(45) not null default ''
);
go
-- clear the log
checkpoint;
go
-- insert rows
insert dbo.Test with (TABLOCK)
(c1)
select top (269) checksum(newid()) --268 will be fully logged.
from master.dbo.spt_values as sv;
go
-- show log entries
SELECT
FD.Operation,
FD.Context,
FD.[Log Record Length],
FD.[Log Reserve],
FD.AllocUnitName,
FD.[Transaction Name],
FD.[Lock Information],
FD.[Description]
FROM sys.fn_dblog(NULL, NULL) AS FD
where fd.AllocUnitName = N'dbo.Test.pk_test_id'
GO
-- count number of fully-logged rows
select
[Fully Logged Rows] = count_big(*)
from sys.fn_dblog(null, null) as fd
where
fd.operation = N'LOP_INSERT_ROWS'
and fd.Context = N'LCX_CLUSTERED'
and fd.AllocUnitName = N'dbo.Test.pk_test_id'
-- LOP_INSERT_ROWS and LCX_CLUSTERED means it's fully logged
-- (despite empty cx and tablock!
go
-- maybe try same thing with tf 610
----------------------------------------------------------------------------------
if object_id('dbo.Test610', 'U') is not null
begin
drop table dbo.Test610;
end;
go
create table dbo.Test610
(
id int not null identity constraint pk_test610_id -- [PK dbo.Test (id)]
primary key,
c1 int not null,
padding char(45) not null default ''
);
go
-- clear the log
checkpoint;
go
-- insert rows
dbcc traceon(610)
insert dbo.Test610 with (TABLOCK)
(c1)
select top (269) checksum(newid())
from master.dbo.spt_values as sv;
go
dbcc traceoff(610)
SELECT
FD.Operation,
FD.Context,
FD.[Log Record Length],
FD.[Log Reserve],
FD.AllocUnitName,
FD.[Transaction Name],
FD.[Lock Information],
FD.[Description]
FROM sys.fn_dblog(NULL, NULL) AS FD
where fd.AllocUnitName = N'dbo.Test610.pk_test610_id'
GO
-- count number of fully-logged rows
select
[Fully Logged Rows] = count_big(*)
from sys.fn_dblog(null, null) as fd
where
fd.operation = N'LOP_INSERT_ROWS'
and fd.Context = N'LCX_CLUSTERED'
and fd.AllocUnitName = N'dbo.Test610.pk_test610_id'
-- SAME THING! 268 FULLY LOGGED ROWS
-- EVEN THOUGH 1. empty table 2. tablock 3. tf(610)
-- look at clustered index insert node in query plan. DMLRequestSort = false, which means that
-- either the estimated row count is less that 250
-- or the calculated data size is less than 2 8K pages
--=======================
-- not that interesting, i don't really care about minimal logging with data sizes this small
-- however.
-- LOW CARDINALITY ESTIMATES can mess you up
if object_id('dbo.Test', 'U') is not null
begin
drop table dbo.Test;
end;
go
create table dbo.Test
(
id int not null identity constraint pk_test_id -- [PK dbo.Test (id)]
primary key,
c1 int not null,
padding char(45) not null default ''
);
go
-- clear the log
checkpoint;
go
declare @nrows bigint = 10000
-- insert rows
insert dbo.Test with (TABLOCK)
(c1)
select top (@nrows) checksum(newid())
from master.dbo.spt_values as sv
option(recompile)
go
-- show log entries
SELECT
FD.Operation,
FD.Context,
FD.[Log Record Length],
FD.[Log Reserve],
FD.AllocUnitName,
FD.[Transaction Name],
FD.[Lock Information],
FD.[Description]
FROM sys.fn_dblog(NULL, NULL) AS FD
where fd.AllocUnitName = N'dbo.Test.pk_test_id'
GO
-- count number of fully-logged rows
select
[Fully Logged Rows] = count_big(*)
from sys.fn_dblog(null, null) as fd
where
fd.operation = N'LOP_INSERT_ROWS'
and fd.Context = N'LCX_CLUSTERED'
and fd.AllocUnitName = N'dbo.Test.pk_test_id'
-- it incorrectly estimates 100 rows (why???)
-- this could be a problem because the cardinality info is stored with the plan cache!
-- it will not be re-evauated even if you change the value of @nrows
-- changed @nrows to 1000
-- 2551 FULLY LOGGED ROWS!!!
-- you can change this behavior by doing option(recompile)
-- option(recompile) gives 0 fully logged rows
--========================================
-- ok but WHAT ABOUT HEAPS
-------------------------------------------
CREATE TABLE dbo.TestHeap
(
id integer NOT NULL IDENTITY,
c1 integer NOT NULL,
padding char(45) NOT NULL
DEFAULT ''
);
GO
-- Clear the log
CHECKPOINT;
GO
-- Insert rows
INSERT dbo.TestHeap WITH (TABLOCK)
(c1)
SELECT TOP (897)
CHECKSUM(NEWID())
FROM master.dbo.spt_values AS SV;
GO
-- Show log entries
SELECT
FD.Operation,
FD.Context,
FD.[Log Record Length],
FD.[Log Reserve],
FD.AllocUnitName,
FD.[Transaction Name],
FD.[Lock Information],
FD.[Description]
FROM sys.fn_dblog(NULL, NULL) AS FD;
GO
-- Count the number of fully-logged rows
SELECT
[Fully Logged Rows] = COUNT_BIG(*)
FROM sys.fn_dblog(NULL, NULL) AS FD
WHERE
FD.Operation = N'LOP_INSERT_ROWS'
AND FD.Context = N'LCX_HEAP'
AND FD.AllocUnitName = N'dbo.TestHeap';
-- wait i thought having (tablock) and an empty table were sufficient for minimal logging?
-- why on earth would insert into a heap table be fully logged??
-- oh, same thing. it has to do with data size for heaps (threshold is 8 8k pages)
-- don't care that much, we'd always be going over the data size of