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266
votes
4answers
286k views

Why Does the Transaction Log Keep Growing or Run Out of Space?

This one seems to be a common question in most forums and all over the web, it is asked here in many formats that typically sound like this: In SQL Server - What are some reasons the ...
132
votes
5answers
193k views

Possible to make MySQL use more than one core?

I've been presented with some dedicated MySQL servers that never use more than a single core. I'm more developer than DBA for MySQL so need some help Setup The servers are quite hefty with an OLAP/...
248
votes
10answers
246k views

What are the main differences between InnoDB and MyISAM?

What are the main differences between InnoDB and MyISAM?
40
votes
2answers
35k views

Configuring PostgreSQL for read performance

Our system writes a lots of data (kind of Big Data system). Write performance is good enough for our needs but read performance is really too slow. The primary key (constraint) structure is similar ...
30
votes
4answers
15k views

How to move a database from SQL Server 2012 to SQL Server 2005

What are my options if I need to move a database from SQL Server 2012(32bit) to SQL Server 2005(64bit)? I know I cannot: restore a backup of the database on SQL Server 2005 detach & attach I ...
87
votes
3answers
31k views

Is a composite index also good for queries on the first field?

Let's say I have a table with fields A and B. I make regular queries on A+B, so I created a composite index on (A,B). Would queries on only A also be fully optimized by the composite index? ...
69
votes
8answers
47k views

MAXDOP setting algorithm for SQL Server

When setting up a new SQL Server, I use the following code to determine a good starting point for the MAXDOP setting: /* This will recommend a MAXDOP setting appropriate for your machine's NUMA ...
59
votes
6answers
70k views

About single threaded versus multithreaded databases performance

H2 is a single threaded database with a good reputation regarding performance. Other databases are multi-threaded. My question is: when does a multi-thread database become more interesting than an ...
38
votes
2answers
58k views

Multi cores and MySQL Performance

Importance of RAM is an established fact but far less material is available about the importance of cores and multithreading when it comes to the usage of CPU by MySQL. I am talking about the ...
60
votes
1answer
20k views

What event information can I get by default from SQL Server?

I often see questions where people want to know if a certain thing happened, or when it happened, or who performed the action. In a lot of cases, SQL Server just doesn't track this information on its ...
23
votes
1answer
45k views

Upgrade SQL Server 2000 database to 2008 R2 and enable new features

I have recently upgraded SQL Server 2000 database to 2008 R2. What I did was: Shutdown SQL Server 2000 (express) service on old machine, Move datafiles (mydatabase.mdf and mydatabase.ldf) to new ...
98
votes
8answers
136k views

Pattern matching with LIKE, SIMILAR TO or regular expressions in PostgreSQL

I had to write a simple query where I go looking for people's name that start with a B or a D : SELECT s.name FROM spelers s WHERE s.name LIKE 'B%' OR s.name LIKE 'D%' ORDER BY 1 I was wondering ...
176
votes
7answers
212k views

How can I optimize a mysqldump of a large database?

I have a symfony application with an InnoDB database that is ~2GB with 57 tables. The majority of the size of the database resides in a single table (~1.2GB). I am currently using mysqldump to ...
106
votes
5answers
178k views

How to safely change MySQL innodb variable 'innodb_log_file_size'?

So I'm fairly new to tuning InnoDB. I'm slowly changing tables (where necessary) from MyIsam to InnoDB. I've got about 100MB in innodb, so I increased the innodb_buffer_pool_size variable to 128MB: ...
87
votes
4answers
65k views

Measure the size of a PostgreSQL table row

I have a PostgreSQL table. select * is very slow whereas select id is nice and quick. I think it may be that the size of the row is very large and it's taking a while to transport, or it may be some ...
29
votes
3answers
6k views

What is a deterministic method for evaluating a sensible buffer pool size?

I'm trying to come up with a sane way to understand if the max server memory (mb) setting is appropriate (either should be lower, or higher, or stay the way it is). I am aware that max server memory (...
59
votes
3answers
68k views

Find highest level of a hierarchical field: with vs without CTEs

note: this question has been updated to reflect that we are currently using MySQL, having done so, I would like to see a how much easier it would be if we switched to a CTE-supporting database. I ...
35
votes
4answers
12k views

I Need to Shrink My Database - I just freed a lot of space

This question is asked in various forms here but the question boils down to: I know shrinking a database is risky. In this case, I've removed so much data and I'll never use it again. How can I ...
16
votes
3answers
5k views

Problem with MySQL subquery

Why does this query DELETE FROM test WHERE id = ( SELECT id FROM (SELECT * FROM test) temp ORDER BY RAND() LIMIT 1 ); sometimes delete 1 row, ...
49
votes
3answers
82k views

How do I get back some deleted records?

I mistakenly deleted around 2,000,000 records from a remote SQL Server 2008 table. The server is not granting me access to the backup files on the server side. Is there any way to get back these ...
33
votes
1answer
8k views

Would index lookup be noticeably faster with char vs varchar when all values are 36 chars

I have a legacy schema (disclaimer!) that uses a hash-based generated id for the primary key for all tables (there are many). An example of such an id is: 922475bb-ad93-43ee-9487-d2671b886479 There ...
89
votes
6answers
54k views

Retrieving n rows per group

I often need to select a number of rows from each group in a result set. For example, I might want to list the 'n' highest or lowest recent order values per customer. In more complex cases, the ...
55
votes
3answers
67k views

Why does InnoDB store all databases in one file?

It was convenient that MyISAM used to store each table in a corresponding file. InnoDB has made advancements in many aspects, but I wonder why InnoDB stores all databases in one file (ibdata1 by ...
39
votes
3answers
8k views

difference in execution plans on UAT and PROD server

I want to understand why there would be such a huge difference in execution of the same query on UAT (runs in 3 sec) vs PROD (run in 23 secs). Both UAT and PROD are having exactly data and indexes. ...
177
votes
4answers
290k views

How large should be mysql innodb_buffer_pool_size?

I have a busy database with solely InnoDB tables which is about 5GB in size. The database runs on a Debian server using SSD disks and I've set max connections = 800 which sometimes saturate and grind ...
76
votes
4answers
27k views

Why is my query suddenly slower than it was yesterday?

[Salutations] (check one) [ ] Well trained professional, [ ] Casual reader, [ ] Hapless wanderer, I have a (check all that apply) [ ] query [ ] stored procedure [ ] database thing maybe that ...
40
votes
5answers
67k views

How do you tune MySQL for a heavy InnoDB workload?

Assuming a production OLTP system with predominantly InnoDB tables What are the common symptoms of a mistuned/misconfigured system? What configuration parameters do you most commonly change from ...
10
votes
1answer
8k views

Why mysql 5.5 slower than 5.1 (linux,using mysqlslap)

my.cnf (5.5 and 5.1 is the same) : back_log=200 max_connections=512 max_connect_errors=999999 key_buffer=512M max_allowed_packet=8M table_cache=512 sort_buffer=8M read_buffer_size=8M thread_cache=8 ...
67
votes
4answers
163k views

How to identify which query is filling up the tempdb transaction log?

I would like to know how to identify the exact query or stored proc which is actually filling up the transactional log of TEMPDB database.
30
votes
1answer
18k views

SQL injection in Postgres functions vs prepared queries

In Postgres, are prepared queries and user defined functions equivalent as a mechanism for guarding against SQL injection? Are there particular advantages in one approach over the other?
28
votes
4answers
15k views

MySQL InnoDB crash post-mortem

MySQL crashed on me this morning. With the exception of the standard MySQL included databases everything I use is InnoDB. I attempted to restart the MySQL daemon but it failed twice. I then ...
22
votes
3answers
12k views

How to have a one-to-many relationship with a privileged child?

I want to have a one-to-many relationship in which for each parent, one or zero of the children is marked as a “favorite.” However, not every parent will have a child. (Think of the parents as ...
11
votes
2answers
6k views

How to model an entity type that can have different sets of attributes?

I’m having some trouble in recreating a database with a one-to-many (1:M) relationship between Users and Items. This is pretty straightforward, yes; however, each Item belongs to a certain Category (...
455
votes
2answers
235k views

What's the difference between a temp table and table variable in SQL Server?

This seems to be an area with quite a few myths and conflicting views. So what is the difference between a table variable and a local temporary table in SQL Server?
129
votes
12answers
62k views

Should binary files be stored in the database?

What is the best place for storing binary files that are related to data in your database? Should you: Store in the database with a blob Store on the filesystem with a link in the database Store in ...
44
votes
5answers
63k views

When is it OK to shrink a Database?

I know shrink is the devil: It reverses page order and is responsible for skin cancer, data fragmentation, and global warming. The list goes on... That being said, say I have a 100 GB database and I ...
29
votes
5answers
94k views

Restoring a backup to an older version of SQL Server

When trying to restore a backup to a SQL Server Express 2008 database I got the following error: Restore failed for Server '...\SQLEXPRESS'. (Microsoft.SqlServer.SmoExtended) System.Data.SqlClient....
12
votes
2answers
6k views

Will these two queries result in a deadlock if executed in sequence?

This is almost certainly the cause of my other question, but I thought it was worth separating the two as I have a hypothesis based on the following log that I would love to have falsified or verified....
101
votes
4answers
96k views

Optimizing queries on a range of timestamps (two columns)

I use PostgreSQL 9.1 on Ubuntu 12.04. I need to select records inside a range of time: my table time_limits has two timestamp fields and one integer property. There are additional columns in my ...
59
votes
3answers
65k views

What is the performance impact of using CHAR vs VARCHAR on a fixed-size field?

I have an indexed column that stores an MD5 hash. Thus, the column will always store a 32-character value. For whatever reason, this was created as a varchar rather than a char. Is it worth the ...
27
votes
2answers
9k views

How do I properly perform a MySQL bake-off?

I want to performance test (aka bake-off) MySQL server rpm against some other forks such as Percona server, MariaDB, and possibly some others. I'm hoping that by asking this question I can better ...
16
votes
2answers
7k views

Trouble deciphering a deadlock in an innodb status log

We are accessing MySQL from the Microsoft ADO.NET connector. Occasionally we are seeing the following deadlock in our innodb status and haven't been able to identify the cause of the problem. It ...
125
votes
8answers
51k views

Why shouldn't we allow NULLs?

I remember reading this one article about database design and I also remember it said you should have field properties of NOT NULL. I don't remember why this was the case though. All I can seem to ...
70
votes
5answers
12k views

Is there a name for this database schema of key values?

We process a routine data feed from a client who just refactored their database from a form that seems familiar (one row per entity, one column per attribute) to one that seems unfamiliar to me (one ...
47
votes
2answers
39k views

What is the actual behavior of compatibility level 80?

Could somebody provide me with a better insight about the compatibility mode feature? It is behaving different than I expected. As far as I understand compatibility modes, it is about the ...
22
votes
3answers
8k views

How is LIKE implemented?

Can anyone explain how the LIKE operator is implemented in current database systems (e.g. MySQL or Postgres)? or point me to some references that explain it? The naive approach would be to inspect ...
76
votes
5answers
27k views

Working of indexes in PostgreSQL

I have a couple of questions regarding working of indexes in PostgreSQL. I have a Friends table with the following index: Friends ( user_id1 ,user_id2) user_id1 and user_id2 are foreign keys to ...
49
votes
4answers
33k views

Function Performance

Coming from a MySQL background, where stored procedure performance (older article) and usability are questionable, I am evaluating PostgreSQL for a new product for my company. One of the things I ...
37
votes
10answers
243k views

restore table from .frm and .ibd file?

I have previously saved a copy of /var/lib/mysql/ddms directory ("ddms" is the schema name). Now I installed a new MySQL on a freshly installed Ubuntu 10.04.3 LTS by running apt-get install mysql-...
29
votes
4answers
3k views

Can spatial index help a “range - order by - limit” query

Asking this question, specifically for Postgres, as it has good supoort for R-tree/spatial indexes. We have the following table with a tree structure (Nested Set model) of words and their frequencies:...

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