Questions tagged [database-internals]

For technical questions about the internal workings of the database engine.

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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?
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0 votes
0 answers
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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 ...
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 ...
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 ...
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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 ...
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 ...
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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 ...
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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 ...
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0 votes
1 answer
116 views

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 ...
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6 votes
1 answer
500 views

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....
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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 ...
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0 votes
2 answers
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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 ...
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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 ...
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4 votes
1 answer
288 views

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 ...
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1 vote
2 answers
133 views

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 ...
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"...
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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 views

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.
2 votes
2 answers
214 views

How memory limit for backup process is calculated?

How "memory limit" for backup process is actually calculated?
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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 ...
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 ...
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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') ...
0 votes
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 votes
0 answers
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 ...
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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)?
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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] ...
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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 ...
1 vote
1 answer
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 ...
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 ...
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 ...
2 votes
1 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 (...
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0 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 ...
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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, ...
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 ...
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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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5 votes
1 answer
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 ...
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 ...
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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 ...
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1 vote
0 answers
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 ...
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 ...
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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 ...
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