Are we able to perform SQL operations like INSERT, UPDATE, DELETE to Database Catalog (It is more theory question than practice)
If a database supports INFORMATION_SCHEMA and provides instruments for altering the database catalog, then yes, you can use SQL operations normally.
For example, in PostgreSQL documentation you can read:
The system catalogs are the place where a relational database management system stores schema metadata, such as information about tables and columns, and internal bookkeeping information. PostgreSQL's system catalogs are regular tables. You can drop and recreate the tables, add columns, insert and update values, and severely mess up your system that way. Normally, one should not change the system catalogs by hand, there are always SQL commands to do that. (For example, CREATE DATABASE inserts a row into the pg_database catalog — and actually creates the database on disk.)
So, you change the catalog indirectly creating a new database. Nonetheless, with PostgreSQL you can directly change the catalog, using SQL commands like DROP, INSERT, UPDATE and so on.
Some RDBMS don't provide such a possibility, such as Oracle Database, IBM DB2, SQLite or Sybase ASE. Some RDBMS provide INFORMATION_SCHEMA, but it is read-only, so you can't do anything crazy, for example, MySQL. Its documentation reads:
Although you can select INFORMATION_SCHEMA as the default database with a USE statement, you can only read the contents of tables, not perform INSERT, UPDATE, or DELETE operations on them.
Related
I'm looking for a way to get all table creation and alteration queries attached to a database, in SQL Server 2000. Is this stored in a system table, or is there a built in method to remake them?
Goal: to extract the schema for customizable backups.
My research so far turned up nothing. My Google-Fu is weak...
Note that I don't know that there's a way to specify which filegroup a stored procedure is on (other than the default). So what you may consider, in order to at least keep the script repository backup small, is:
create a filegroup called non_data_objects, and make it the default (instead of PRIMARY).
create a filegroup for each set of tables, and create those tables there.
backup each set of tables by filegroup, and always include a backup of non_data_objects so that you have the current set of procedures, functions etc. that belong to those tables (even though you'll also get the others). Because 1. will only contain the metadata for non-data, it should be relatively small.
You might also consider just using a different database for each set of tables. Other than using three-part naming in your scripts that need to reference the different sets, there really is no performance difference. And this makes your backup/recovery plan much simpler.
I am working on a program that issues DDL. I would like to know whether CREATE TABLE and similar DDL can be rolled back in
Postgres
MySQL
SQLite
et al
Describe how each database handles transactions with DDL.
http://wiki.postgresql.org/wiki/Transactional_DDL_in_PostgreSQL:_A_Competitive_Analysis provides an overview of this issue from PostgreSQL's perspective.
Is DDL transactional according to this document?
PostgreSQL - yes
MySQL - no; DDL causes an implicit commit
Oracle Database 11g Release 2 and above - by default, no, but an alternative called edition-based redefinition exists
Older versions of Oracle - no; DDL causes an implicit commit
SQL Server - yes
Sybase Adaptive Server - yes
DB2 - yes
Informix - yes
Firebird (Interbase) - yes
SQLite also appears to have transactional DDL as well. I was able to ROLLBACK a CREATE TABLE statement in SQLite. Its CREATE TABLE documentation does not mention any special transactional 'gotchas'.
PostgreSQL has transactional DDL for most database objects (certainly tables, indices etc but not databases, users). However practically any DDL will get an ACCESS EXCLUSIVE lock on the target object, making it completely inaccessible until the DDL transaction finishes. Also, not all situations are quite handled- for example, if you try to select from table foo while another transaction is dropping it and creating a replacement table foo, then the blocked transaction will finally receive an error rather than finding the new foo table. (Edit: this was fixed in or before PostgreSQL 9.3)
CREATE INDEX ... CONCURRENTLY is exceptional, it uses three transactions to add an index to a table while allowing concurrent updates, so it cannot itself be performed in a transaction.
Also the database maintenance command VACUUM cannot be used in a transaction.
Can't be done with MySQL it seems, very dumb, but true... (as per the accepted answer)
"The CREATE TABLE statement in InnoDB is processed as a single
transaction. This means that a ROLLBACK from the user does not undo
CREATE TABLE statements the user made during that transaction."
https://dev.mysql.com/doc/refman/5.7/en/implicit-commit.html
Tried a few different ways and it simply won't roll back..
Work around is to simply set a failure flag and do "drop table tblname" if one of the queries failed..
Looks like the other answers are pretty outdated.
As of 2019:
Postgres has supported transactional DDL for many releases.
SQLite has supported transactional DDL for many releases.
MySQL has supported Atomic DDL since 8.0 (which was released in 2018).
While it is not strictly speaking a "rollback", in Oracle the FLASHBACK command can be used to undo these types of changes, if the database has been configured to support it.
DDLs and DMLs are two strict categories of types of statements used for interacting with a database. I am not sure why this categorization exists.
Is there a fundamental difference in the way an Oracle database would work internally with respect to a DDL and DML statement?
One major (technical) difference between DDL and DML in Oracle is, that DDL is not transactional, i.e. they cannot be rolled back and don't require a commit. As a matter of fact DDL in Oracle does an implicit commit before it's executed.
Other databases (e.g. Postgres, DB2) do not make a difference with regards to transactions between DDL and DML
After all it's just a categorization, similar to the terms "application" and "server" (as in database server). From an operating point of view, OpenOffice and Oracle are both simply "applications", but yet we classify them into different categories.
DDL statements are used to define database structures, objects, and schemas whereas DML statements are used for managing data within schema objects. At the end of the day, Oracle (o r any other data management system) would process each type statement according to security permissions and object availability (i.e. locks on tables / views and isolation levels).
Also, schema definitions are held in internal master tables so your DDL statements actually affect the data stored in those tables and perhaps can be considered "master DML" statements in that sense.
If your question amounts to "is there a reason why it is necessary for DDL and DML to "be implemented differently", the answer is "NO".
However, the definers of the SQL language have opted for making DDL syntactically distinct. As a consequence, adding a column to a table must be done through the appropriate ALTER TABLE command. A side-effect of that command is that a row gets inserted in the catalog table that documents all columns. Stress side-effect.
But there is no fundamental reason why the insertion of a row in the catalog table could not be the trigger itself for the column addition, thus entirely eliminating the need for any "dedicated DDL".
In Sqlsever Enterprise manager, there are some default databases are provided like tempdb and etc. What is significance of those databases?
TempDB is used for temporary work in SQL Server. Anytime you create a temp table that storage is done inside of TempDB. Here is a very good article from MSDN
Here are some points referenced from the MSDN:
The tempdb system database is a global resource that is available to all users connected to the instance of SQL Server and is used to hold the following:
•Temporary user objects that are explicitly created, such as: global or local temporary tables, temporary stored procedures, table variables, or cursors.
•Internal objects that are created by the SQL Server Database Engine, for example, work tables to store intermediate results for spools or sorting.
•Row versions that are generated by data modification transactions in a database that uses read-committed using row versioning isolation or snapshot isolation transactions.
•Row versions that are generated by data modification transactions for features, such as: online index operations, Multiple Active Result Sets (MARS), and AFTER triggers.
Operations within tempdb are minimally logged. This enables transactions to be rolled back. tempdb is re-created every time SQL Server is started so that the system always starts with a clean copy of the database. Temporary tables and stored procedures are dropped automatically on disconnect, and no connections are active when the system is shut down. Therefore, there is never anything in tempdb to be saved from one session of SQL Server to another. Backup and restore operations are not allowed on tempdb.
There is also the master database (holds information about all databases), Model database, MSDB (stores information on the sql agent, dts, jobs, etc).
More info here as well
MASTER - This keeps all server-level information, and meta-data about all databases on the server. Don't lose this :)
MSDB - Holds information about SQL Agent jobs and job run history
TEMPDB - Used as a temporary "work space" for temporary tables and lots of other stuff (like sorting and grouping)
MODEL - When you create a new, blank database, it makes a copy of MODEL as a template
DISTRIBUTION - (You will only see this on servers where you have set up replication) Holds records pending for replication.
SQL Server uses tempdb to store internal objects such as the intermediate results of a query. You can get more details here.
I am trying to understand the tempDB and following are the doubts popping in my mind.
What is the lifetime of data in tempDB? Say a query is doing some Order By and uses tempDB for performing that. After this query finishes, someone else also executes a query which utilizes the tempDB. Will the second query find records written by first query in the tempDB or will they be deleted?
Are there any visible tables created inside the tempDB by the Sql Engine? How can I know which temporary table is created because of this query? Is there any naming convention followed by the Sql engine for naming these temporary tables?
I am new to tempDB so please pardon me for asking such silly (if at all) questions :-)
It will be very nice if someone can point me to a good resource which can help me learn about tempDB.
Temp table is stored in tempdb until the connection is dropped (or in the case of a global temp tables when the last connection using it is dropped). You can also (and it is a good practice to do so) manually drop the table when you are finished using it with a drop table statement.
No, others cannot see your temp tables if they are local temp tables (They can see and use global temp tables) Multiple people can run commands which use the same temp table name but they will not be overlapping in a local temp table and so you can have a table named #test and so can 10,000 other users, but each one has its own structure and data.
You don't want to generally look up temp tables in tempdb. It is possible to check for existence, but that is the only time I have ever referenced tempdb directly. Simply use your temp table name. Example below of checking for existence
IF OBJECT_ID('TempDB.dbo.#DuplicateAssignments') IS NOT NULL
BEGIN
DROP TABLE #DuplicateAssignments
END
You name temp tables by prefacing the name with # (for local tables the ones you would use 999.9% of the time) and ## for global temp tables, then the rest of the name you want.
There's a few MSDN articles that are probably the best source of information on the tempDB database in SQL Server.
tempdb Database
The tempdb system database is a global
resource that is available to all
users connected to the instance of SQL
Server and is used to hold the
following:
Temporary user objects that are explicitly created, such as: global or
local temporary tables, temporary
stored procedures, table variables, or
cursors.
Internal objects that are created by the SQL Server Database Engine, for
example, work tables to store
intermediate results for spools or
sorting.
Row versions that are generated by data modification transactions in a
database that uses read-committed
using row versioning isolation or
snapshot isolation transactions.
Row versions that are generated by data modification transactions for
features, such as: online index
operations, Multiple Active Result
Sets (MARS), and AFTER triggers.
Operations within tempdb are minimally
logged. This enables transactions to
be rolled back. tempdb is re-created
every time SQL Server is started so
that the system always starts with a
clean copy of the database. Temporary
tables and stored procedures are
dropped automatically on disconnect,
and no connections are active when the
system is shut down. Therefore, there
is never anything in tempdb to be
saved from one session of SQL Server
to another. Backup and restore
operations are not allowed on tempdb.
There's also tempdb and Index Creation, this blog post along with Working with tempdb in SQL Server 2005 which states:
The SQL Server system database, tempdb, has undergone a number of changes in SQL Server 2005. There are new tempdb usages and internal optimizations in SQL Server 2005; tempdb architecture is mostly unchanged since SQL Server 2000.
The tempdb system database is very similar to a user database. The main difference is that data in tempdb does not persist after SQL Server shuts down.
The temporary tables created in TempDB are dropped when the query is completed.
I'm not sure on this (I would have to try it), but I think theoretically ALL tables created in TempDB are visible, although only the user that created the table has permission to access it.