Questions tagged [database-internals]
For technical questions about the internal workings of the database engine.
222
questions
1
vote
2
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views
Is space reserved for a field when data doesn't use the full length?
A column is marked as nvarchar(50), but the value stored is only two characters. Do the remaining 48 characters take up space?
- 19
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0
answers
27
views
How does ghost cleanup process work internally? [duplicate]
I wanted to understand when a record is updated/deleted in 8-kb page, what happens to old record?
I know it gets cleaned up by ghost cleanup process - but what method does it use for this?
Some ...
- 133
3
votes
2
answers
200
views
Logical reads and LOB logical reads
I have a query app scanning a whole table with a text field.
The query is doing this many reads:
Scan count 1, logical reads 170586, physical reads 3, read-ahead reads
174716, lob logical reads ...
- 602
9
votes
1
answer
510
views
SQL Server LOB variables and memory usage
When I use a variable of a large object (LOB) data type in SQL Server, is the whole thing kept in memory at all times? Even if it is 2GB in size?
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4
votes
1
answer
134
views
How does SQL estimate the number of rows in a less than < Predicate
I have been doing some testing to try to better understand how SQL Server uses a histogram to estimate the number of rows that will match an equality predicate and also a < or > predicate
Given ...
- 1,600
2
votes
1
answer
141
views
Diff backup fails with error 3035 but log backup succeeds
We have a SQL Server 2014 Enterprise where the DIFF backups fail.
This is the error message we get:
Msg 3035, Level 16, State 1, Server sqltest, Line 1
Cannot perform a differential backup for ...
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2
votes
1
answer
96
views
Why Does SQL Server not have 200 buckets in the statistics histogram when there are >100k distinct values in the table
Given I am using the AdventureWorks2016 OLTP database why does the statistics histogram for the index PK_TransactionHistory_TransactionID on table Production.TransactionHistory only contain 3 ...
- 1,600
3
votes
1
answer
77
views
Change the definition of an Index with Expression without dropping/recreating the index
I'm in a peculiar situation:
I have multiple indexes with expressions that use a function. I want to switch them to a different function, where I can guarantee the function behaves the same (so the ...
- 207
1
vote
2
answers
28
views
Does any RDBMS "inline" a 1:0-1 relationship's dependent table?
In the case of a 1:0-1 relationship between a principal table p and dependent table d, when commonly-used queries will read from both p and d, do any RDMBs inline d to eliminate the need to store d ...
- 481
0
votes
1
answer
116
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What is difference between checkpoint_timeout and checkpoint_completion_target in PostgreSQL?
I am MSSQL guy and I find it a bit difficult to understand the main purpose of checkpoint_completion_target. I cannot find a comprehensive resource that would explain more clearly the difference ...
- 1,193
6
votes
1
answer
500
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Bitmap Creation in Execution Plan Causes bad Estimate on Clustered Index Scan
Given the following simple query on the StackOverflow2010 database:
SELECT u.DisplayName,
u.Reputation
FROM Users u
JOIN Posts p
ON u.id = p.OwnerUserId
WHERE u....
- 1,600
16
votes
1
answer
484
views
Does SQL Server 2019 have task scheduling differences between enterprise and standard editions?
In How It Works: SQL Server 2012 Database Engine Task Scheduling, Bob Dorr explains some of the changes for worker scheduler assignment in SQL Server 2012. He mentions that some of the improvements ...
- 30.9k
0
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2
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54
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how do modern data warehouses tackle frequent small writes? esp. when streaming data is one of the sources? [closed]
So for many days, I had a question in mind.
How do modern data warehouses tackle frequent small writes? esp. when streaming data is one of the sources?
e.g. Kafka/Kinesis => DW(Snowflake, Teradata, ...
0
votes
1
answer
137
views
Reading of fixed-length data types
Say I have a fixed length column and I am SELECTing from it, say 100 rows. When reading different rows of the fixed length column, does SQL Server check the length of the column for every row or does ...
- 655
2
votes
1
answer
88
views
Why does SQL Server Trace Flag 715 behave differently from the TABLOCK query hint?
It's my understanding, based on numerous sources (including this one: https://techcommunity.microsoft.com/t5/sql-server/migrating-sap-workloads-to-sql-server-just-got-2-5x-faster/ba-p/384910) that SQL ...
- 135
4
votes
1
answer
288
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Suggestion for nearly gap-less sequences in postgres
In PostgreSQL sequences are designed to have gaps for reasons mentioned in this post https://stackoverflow.com/questions/9984196/postgresql-gapless-sequences. My question is: could sequences not be ...
- 237
1
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2
answers
133
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Tracing Postgres frontend and backend messages
As per my understanding when we execute a command or query, the Postgres client sends a frontend message, and in return gets the response in the backend message format.
How do I capture and check ...
- 135
0
votes
0
answers
59
views
B-tree indexing example
I am trying to understand "how" Postgres creates the indexes using b-trees with an example.
To be specific, I am looking to understand how the b-tree will look like when the ("Name"...
- 109
6
votes
3
answers
836
views
Does a SQL Server update nvarchar statement overwrite the same address on disk if the new value is the same size?
Is it possible to overwrite the same disk address in a T-SQL update statement? A use case would be preventing recovery of the original text on disk for security or anonymity reasons.
CREATE TABLE Test(...
3
votes
1
answer
117
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Why is log reserve negative in the SQL Server compensation log when using fn_dblog command
I was using the fn_dblog command and was not able to get why the log reserve was negative.
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2
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2
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214
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How memory limit for backup process is calculated?
How "memory limit" for backup process is actually calculated?
- 1,193
2
votes
2
answers
184
views
Difference between avg Disk sec/write and io_stall_write_ms
We have a SQL Server 2016 SP2 Enterprise with latest CU with the database files spread out over different disks.
So we have data,log, tempdb and system db's have each their own drive. Date and log ...
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12
votes
1
answer
529
views
Prevent THREADPOOL waits due to idle worker thread trimming
After reading Unusual THREADPOOL Waits by Josh Darnell, a Twitter user mentioned there is an undocumented trace flag to prevent trimming idle workers:
The idea is that once SQL Server has created ...
- 73.8k
1
vote
1
answer
103
views
Storage space requirements for a varchar(40) column to store 'abcd' with charset 'ucs2' and 'utf8mb4'?
I understand that if the length is > 255 it will require 2 bytes to store the length prefix. I just need to verify my assumptions. Please tell me the length prefix and the length of string('abcd') ...
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0
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0
answers
16
views
Array of chars? [duplicate]
Can we create an array of chars in postgresql i.e (Column_name char[5]) and does it take only 5 bytes in disk ? since type char takes only one byte
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0
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0
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20
views
BIT VARYING internals
I Have a table called IP_Block in which I created two columns
ID of type Bit Varying(32) as PK
IP Bloc of type CIDR
the construction of the ID is the following :
For example for the block 1.0.0.0/8 ...
- 35
1
vote
2
answers
2k
views
varchar(n) size?
When I use varchar(5) in a INSERT query it means that the attribute in the table will take exactly 5 bytes in memory? (Given that one printable character takes one byte)?
- 35
14
votes
2
answers
1k
views
Unclear update conflict
I have two questions:
1. Why do I get update conflict in this situation instead of just blocking:
-- prepare
drop database if exists [TestSI];
go
create database [TestSI];
go
alter database [TestSI] ...
- 183
3
votes
1
answer
209
views
Badly-formed histogram causes bad estimates on Nested Loop
We have a query on a SQL Server 2016 SP2 CU12 Enterprise where the Query Optimizer estimates that only 1 row would come out of the Nested Loops Join operator, in reality 108501 rows came back. This ...
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1
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1
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275
views
what is difference between uniform extents and mixed extents in Database file storage structure?
I just went to some resources saying:
uniform extents are owned by single object and mixed extents can be shared by maximum 8 objects.
But I'm unable to grasp their insight meaning, like what is the ...
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1
vote
0
answers
66
views
My question is in a select statement how do you return the RID along with the table data?
I need your help with a concept I want to show developers, that concept is to help them understand how SQL Server retrieves data from a table.
In the event that a developer does not use a clustered ...
- 11
9
votes
2
answers
565
views
EXCEPT & INTERSECT: Purpose of Passive Projection in Logical Plan
The following EXCEPT query produces a logical plan with a seemingly purposeless projection. This also occurs for INTERSECT.
What is the purpose of the projection? e.g. Is there a different EXCEPT ...
- 879
2
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answer
189
views
SQLite Internals - Records
Hey I'm trying to wrap my head around SQLite data storage, specifically how it is storing Records. I've found a book The Definitive guide to SQLlite, where the author explains internal record format (...
- 121
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votes
2
answers
402
views
What triggers the SSRS to generate a log file or modify a current one?
When running a trace on my new SSRS Reporting Services 2016, after troubleshooting some problems related to migrating from SSRS 2008, I find the following pattern running continuously:
Here is the ...
4
votes
2
answers
2k
views
Difference between Physical Reads and Read-Ahead Reads
I am trying to understand read-ahead reads, but it seems a bit complicated to me. I searched on the web and got the following:
From Reading Pages (Microsoft documentation):
Read-ahead anticipates the ...
- 1,193
0
votes
0
answers
536
views
What means execute S_2, S_3 and S_4: COMMIT in Postgresql logs?
I'd noticed different commit types in Postgres log:
2019-12-20 12:00:00 [99999]: [12-1] host=...,user=...,db=... LOG: duration: 1000.000 ms execute S_2: COMMIT
2019-12-20 12:00:00 [99999]: [12-1] ...
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4
votes
1
answer
272
views
SQL Server chooses unselective index
I was testing SQL Server indexes and found very strange behavior. Here is my code:
DROP TABLE IF EXISTS dbo._Test
DROP TABLE IF EXISTS dbo._Newtest
GO
CREATE TABLE _Test(
ID INT NOT NULL,
...
- 1,343
0
votes
1
answer
199
views
does automatic checkpoint flushes dirty pages which are related to un-committed transactions to MDF File in SQL Server
when an automatic checpoint runs in background in sql server, i know it flushes all the dirty pages to data file.
does it also flushes dirty pages which are related to un-committed transactions into ...
0
votes
0
answers
193
views
Trimming copy/delete mysql table rows using Trigger
I am using Freeradius AAA server on Ubuntu 18 server which uses mySQL ver 5.7.27 as DB to store entries. INNODB is being used. It have a table called radacct which stores users accounting /session ...
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17
votes
1
answer
481
views
Constant Scan applying
This question is a continuation of optimizer behavior exploration in regards to VALUES construct started here and here. I want to ask about VALUES and APPLY this time.
Using CROSS APPLY to alias an ...
- 2,314
1
vote
1
answer
92
views
Physical co-clustering of two or more logical relations/tables in modern RDBMS
One possible approach to optimize search queries is (a) to store the records that retain data that corresponds to different relations/tables in (b) the same file → on the same pages. That way, a join ...
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1
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446
views
Constant Scan joining
In preparation of my previous Constant Scan question I was experimenting with VALUES in various ways and came across the thing regarding joining of VALUES that looks odd to me.
Setup is simple
...
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15
votes
1
answer
512
views
Cardinality estimate outside the histogram
Setup
I'm having some trouble understanding a cardinality estimate. Here's my test setup:
the 2010 version of the Stack Overflow database
SQL Server 2017 CU15+GDR (KB4505225) - 14.0.3192.2
the new ...
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1
vote
0
answers
96
views
Character set issue
Error 'Character set '#255' is not a compiled character set and is not specified in the '/user/share/mysql'.
Facing error in replication channel from mysql 8.0.16 to mysql 5.7.25.Is there any ...
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7
votes
1
answer
484
views
"Invalid Protection Option"
In the following error message, the reason given is "Invalid Protection Option". What does that indicate?
SQL Server detected a logical consistency-based I/O error: invalid protection option
...
- 67.7k
10
votes
1
answer
413
views
Hash aggregate bailout
A question that arose in a chat discussion:
I know hash join bailout switches internally to a sort of nested loops thing.
What does SQL Server do for a hash aggregate bailout (if it can happen at ...
- 73.8k
14
votes
4
answers
1k
views
Index Uniqueness Overhead
I've been having an ongoing debate with various developers in my office on the cost of an index, and whether or not uniqueness is beneficial or costly (probably both). The crux of the issue is our ...
- 243
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0
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35
views
DCM page in Filestream_Data_Filegroup
I'm doing some research about the DCM (Differential Change Map) page in SQL Server 2016.
This is what I discovered so far:
page type: 16
each DCM page covers a GAM interval
each data file has a DCM ...
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7
votes
2
answers
211
views
What is the purpose of this Uniq1002 column in this index scan?
Take the following repro:
USE tempdb;
IF OBJECT_ID(N'dbo.t', N'U') IS NOT NULL
DROP TABLE dbo.t
GO
CREATE TABLE dbo.t
(
id int NOT NULL
PRIMARY KEY
NONCLUSTERED
...
- 67.7k
0
votes
1
answer
555
views
View images as query results [closed]
I've done lots of research on the topic but I'm drawing a blank. Are their any database engines that can return rendered images in the query response?
For example:
Select images from fruit where ...
- 55