Questions tagged [database-internals]
For technical questions about the internal workings of the database engine.
1
vote
0answers
17 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 ...
0
votes
0answers
11 views
Mysql 8.0.16 crash due to informatiin_schema.Keywords table
How to resolve issue of mysql 8.0.16 crash?mysql 8.0.16 crash many times and shown server logs due to invalid argument.and in error log query is select WORD from information_schema.KEYWORDS.
how to ...
7
votes
2answers
123 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
...
0
votes
1answer
25 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 ...
-2
votes
1answer
54 views
SSMS Application Security idle session locking window [closed]
We are running multiple versions of SQL Server Management Studio in our network, I believe from 2000 through 2017. For our security review we need to find out how to set an inactivity timeout in the ...
0
votes
0answers
44 views
Is there a way to make postgres “COUNT WHERE” query log(N)?
Afaik, now this request SELECT COUNT(*) FROM user WHERE rating > 50 looks like this:
find an item with the value of rating 50(or greater) in our b-tree index, pretty fast.
Iterate though all items ...
5
votes
0answers
196 views
Alwayson AG lease timeout and healthcheck timeout
We have a physical SQL Server 2016 Enterprise SP2 with two instances and each instance contains one AG.
Sometimes it happens that one AG goes to resolving (without failover), the cluster resource is ...
1
vote
1answer
66 views
Calculating disk usage with INFORMATION_SCHEMA.TABLE versus file system?
MySQL's database size can be calculated with
SELECT table_schema AS db_name, SUM(data_length + index_length) AS size
FROM INFORMATION_SCHEMA.TABLES
WHERE table_schema != 'INFORMATION_SCHEMA'
GROUP BY ...
2
votes
2answers
133 views
Postgres heap vs SQL Server clustered index
I am transitioning from SQL Server to Postgres, and one of the biggest things for me to digest is the non-existence of the "clustered key" that sorts the data in Postgres.
Can someone share their ...
13
votes
2answers
1k views
Why is a temp table a more efficient solution to the Halloween Problem than an eager spool?
Consider the following query that inserts rows from a source table only if they aren't already in the target table:
INSERT INTO dbo.HALLOWEEN_IS_COMING_EARLY_THIS_YEAR WITH (TABLOCK)
SELECT ...
4
votes
1answer
66 views
SQL Server 2016 still uses mixed_extent_allocation in TempDB
Today I was troubleshooting a problem on TempDB on SQL Server 2016 SP2 using extended events and added the mixed_extent_allocation and transaction_log event to my trace.
I expected ...
4
votes
2answers
479 views
Why does an index rebuild requires a Sch-M lock?
I don't see why an index rebuild would require a Sch-M lock on the given index.
2
votes
2answers
80 views
When does MySQL begin to start sending data to a client?
I am working on MySQL 5.7 with the default configuration of InnoDB storage engine. I have two questions each for a different scenario and need to know the internal working on how MySQL sends data to ...
0
votes
1answer
26 views
Will Postgresql statements execute synchronously if autocommit=true?
If a single application thread sends a series of INSERT statements and autocommit=true, will the statements be executed synchronously on the server?
I read that on the postgres server side, "there ...
2
votes
1answer
69 views
How to find function definition
How can I find the definitions of below system functions in SQL Server:
has_access
permission_name
I tried with DAC connection but getting below error:
Msg 15009, Level 16, State 1, Procedure ...
11
votes
1answer
458 views
Why is this stream aggregate necessary?
Check out this query. It's pretty simple (see the end of the post for table and index definitions, and a repro script):
SELECT MAX(Revision)
FROM dbo.TheOneders
WHERE Id = 1 AND 1 = (SELECT 1);
...
0
votes
2answers
147 views
How to understand Composite Primary Key when it comes to storage (in Mysql)?
I can image how rows will be arranged in a single database page as primary key order (example an auto-increment key) after reading this post. But I wonder how it will in the case of composite primary ...
6
votes
0answers
96 views
dm_os_memory_cache_clock_hands understanding
As I mentioned in this question I'm trying to understand the working of clock hands. Simply put clock hands move when there is memory pressure so that cache entries can be removed or the cost can be ...
2
votes
1answer
101 views
sql server cache clock hands
I'm doing some research about how to troubleshoot local memory pressure. Correct me if I'm wrong but as I understand it is that SQL Server uses a CLOCK algorithm for the cache- and userstores. Each ...
0
votes
2answers
72 views
No 'Copying to tmp table' state in profiling a mysql query
I have a table with 100k rows
CREATE TABLE `small_table` (
`pk` int(11) NOT NULL AUTO_INCREMENT,
`varc` varchar(255) DEFAULT NULL,
`txt` text,
PRIMARY KEY (`pk`)
) ENGINE=InnoDB ...
9
votes
1answer
395 views
Measuring plan eviction
We have a SQL Server 2016 SP1 with max memory set to 24GB.
This server has a high numbers of compiles, only 10% of these compiles are from Ad-Hoc queries. So the newly compiled plans should be stored ...
1
vote
1answer
46 views
How are page splits determined internally in MySQL?
Going through this answer for a stack-overflow question, I found a fact that
In The physical structure of InnoDB index pages I describe the Last Insert Position, Page Direction, and Number of ...
3
votes
1answer
90 views
leaf_allocation_count in dm_db_index_operational_stats not changing on insert bulk
I was doing some index maintenance on a SQL Server 2016 SP2 CU2 and I noticed something I couldn't explain.
I've a clustered index on a table of about 66 million rows and rows are added throughout ...
29
votes
1answer
2k views
Why is a scan faster than seek for this predicate?
I was able to reproduce a query performance issue that I would describe as unexpected. I'm looking for an answer that's focused on internals.
On my machine, the following query does a clustered index ...
0
votes
2answers
141 views
Extent Allocation Mechanism in SQL Server 2017
I was looking at the SQL Server 2017 Documentation for page and extent allocations.
What I got to know from online resources and sessions in SQL conferences: Starting with SQL Server 2016, Uniform ...
11
votes
1answer
110 views
List ROW_OVERFLOW_DATA pages for a specific table
I'm attempting to get a list of pages for a table that has rows with ROW_OVERFLOW_DATA. I can get the list of allocated pages from the undocumented DMV, sys.db_db_database_page_allocations, however, ...
3
votes
2answers
58 views
PostgreSQL GiST compress skeleton when no compression required
I'm trying to implement my GiST index, where storage type is the same as column type (bytea). They are even going to have same length because these are bitarrays and unions are just disjunctions of ...
3
votes
1answer
156 views
How large is SQLites minimum data storage overhead and what generates them?
Motivation:
In my company there exists the (probably strange) idea to store pure time series data in a normal SQL table (instead of using BLOBs or just binary files which would probably be a better ...
2
votes
1answer
129 views
Performance difference in accessing differrent columns in a Postgres Table
Created a table with 200 bigint column, 200 varchar column. (Postgres 10.4)
create table i200c200 ( pk bigint primary key, int1 bigint, int2 bigint,....., int200 bigint, char1 varchar(255),......, ...
9
votes
1answer
292 views
Index Seek Operator Cost
For the AdventureWorks sample database query below:
SELECT
P.ProductID,
CA.TransactionID
FROM Production.Product AS P
CROSS APPLY
(
SELECT TOP (1)
TH.TransactionID
FROM ...
4
votes
2answers
1k views
In what cases are BLOB and TEXT stored in-line on InnoDB?
I've googled this and can't find it, you can see Rick James mention it here.
... BLOB and TEXT are not always stored separately
The answer on What is the difference between MySQL VARCHAR and TEXT ...
4
votes
2answers
243 views
Advantages to column constraints over table constraints
Is there any advantages whatsoever, other than style to writing this,
CREATE TABLE foo (
a int PRIMARY KEY,
b int
);
And then,
CREATE TABLE bar (
a int REFERENCES foo,
c int
);
Over,
...
1
vote
1answer
102 views
Does a backup include dropped, hidden columns still in a table's physical structure?
If I have a table with an NVARCHAR(MAX) column and I drop it, the standard way to reclaim the space it used is to either:
Rebuild the clustered index or
Run DBCC CLEANTABLE
If I didn't perform ...
3
votes
1answer
116 views
PostgreSQL priority between CTE and real table
Is there some kind of priority ranks assigned to CTE and table name ? For instance, if I have a table called table_a in the public schema, and I create a CTE table named table_a using WITH, which ...
2
votes
0answers
55 views
How is page data rearranged after clustered index is created?
Can anyone please explain what happens at the page level when clustered indexes are created? How do pages rearrange data once clustered indexes are created?
8
votes
1answer
304 views
Why does a Set Returning Function (SRF) run slower in a FROM clause?
This is a database-internals question. I'm using PostgreSQL 9.5, I'm wondering why Set Returning Functions (SRFs), also known as Table Valued Functions (TVFs) run slower when in a FROM clause, for ...
0
votes
1answer
60 views
Is file a basic form of database? [duplicate]
I’m having an interesting conversation with my friend who’s a developer. I’m a database admin. I say file is a basic form of database. I believe whatever that stores data can be called a database. He ...
1
vote
1answer
51 views
How can we foresee (and configure) B-Tree and R-Tree indexes width and depth in PostgreSQL? [closed]
The B-Tree and the R-Tree are two similar index structures that PostgreSQL offers in its implementation.
Both structures can have at most m children per node and as other tree structures, children ...
7
votes
1answer
897 views
Internally, how does OpenRowset(TABLE …) work?
I see a lot of internal views, such as sys.syscomments making calls to CROSS APPLY OpenRowset(TABLE, oid). I am wondering how this function operates and what it's reading from. Let's look at one ...
3
votes
1answer
1k views
What is the significance of the principal_id in sys.schemas?
What is the meaning of the principal_id in sys.schemas and when would it ever be different from schema_id?
1> SELECT LEFT(name,20), schema_id, principal_id FROM sys.schemas;
2> GO
...
2
votes
2answers
387 views
What is an “Obfuscation key” in SQL Server?
Jokes aside, what is an "Obfuscation key"? I'm just going off the docs.
sys.sysclsobjs Exists in every database. Contains a row for each classified entity that shares the same common properties ...
2
votes
1answer
177 views
How does the sys.objects Catalog View pull from itself? [duplicate]
I'm using this stored procedure called OBJECT_DEFINITION as was suggested. It's helping me wrap my head around the SQL Server system. However, there is one area that I can't yet figure out. On this ...
5
votes
3answers
1k views
What is the significance of the trailing dollar sign `$` on identifiers in SQL Server?
Dumping some internal views, such as sys.system_objects as done in this question I see that some identifiers end with a dollar-sign $
CREATE VIEW sys.system_objects AS
-- cutout
FROM sys....
6
votes
3answers
5k views
Making sense of sys.objects, sys.system_objects, and sys.sysobjects?
In this question I was writing a query using sys.sysobjects. However, one of the answers mentioned sys.system_objects. I'm just wondering what is the difference between these tables?
sys.objects
sys....
-2
votes
2answers
457 views
What internal functions are undocumented in SQL Server?
I just read about fn_dblog, and fn_dump_dblog. I was wondering how do I know what functions like this exist,
I'm not the only one either, the comments are scattered with people wanting to know more,
...
2
votes
1answer
542 views
What is a SQL Server “lop”?
The tool ldfreader.exe has an option called -lopFilter
-lopFilter Dump only records with these LOP. (short form: -l)
But, I don't see anything explaining what a LOP is. What is known about "...
4
votes
1answer
349 views
Can I extend SQL Server with C++ or C?
So, I created a PE32+ DLL, in an attempt to load it into SQL Server,
$ file example_dll.dll
example_dll.dll: PE32+ executable (DLL) (console) x86-64, for MS Windows
but when I try to compile it as ...
1
vote
2answers
221 views
How do I use SQL Server's “Hekaton”?
The SQL Server 2017 version on Linux comes packaged with a utility called mtlogreader.exe that is absent from the Windows distribution,
MtLog Reader - Dump Hekaton MtLog files
Usage: \\VBOXSVR\mssql\...
2
votes
1answer
326 views
What differences are there with SQL Server 2017's packaging of `Binn` on Linux and Windows?
Knowing that the Binn is packaged differently on Windows and Linux, I'm wondering what differences there are between the two? Do they come with different utilities? On Linux Binn is distributed in a ...
1
vote
1answer
381 views
What are these SFP files that SQL Server distributes?
What are the SFP files that come with SQL Server 2017?
/opt/mssql/lib/sqlservr.sfp
/opt/mssql/lib/sqldkxplat.sfp
/opt/mssql/lib/secforwarderxplat.sfp
/opt/mssql/lib/system.common.sfp
/opt/mssql/lib/...
