Is Commit a DML instruction in PL\SQL?
No, it's a transaction control (TCL) statement, not a data manipulation (DML) statement. Here is a list of SQL statement types.
No COMMIT is not a DML instruction. It comes under TCL. COMMIT is used to save the changes made by using DML instructions. In general COMMIT is used with ROLLBACK.
DML:
Data manipulation language (DML) statements access and manipulate data in existing schema objects. These statements do not implicitly commit the current transaction.
CALL
DELETE
EXPLAIN
PLAN
INSERT
LOCK
TABLE
MERGE
SELECT
UPDATE
The SELECT statement is a limited form of DML statement in that it can only access data in the database. It cannot manipulate data in the database, although it can operate on the accessed data before returning the results of the query.
The CALL and EXPLAIN PLAN statements are supported in PL/SQL only when executed dynamically. All other DML statements are fully supported in PL/SQL
TCL:
Transaction control statements manage changes made by DML statements
COMMIT
ROLLBACK
SAVEPOINT
SET TRANSACTION
Related
Does Bigquery support single DML(insert, update, delete) SQL statement rollback?
In other words, is there an option to make a single DML statement be an auto-commit transaction that supports rollback when it fails?
Thank you for your kind reply in advance.
Steve.
There is a new feature in BigQuery that supports roll-back when a multi-statement transactions. Take a look at the official docs. It isn't in GA yet.
BigQuery supports multi-statement transactions inside scripts. A multi-statement transaction lets you perform mutating operations, such as inserting or deleting rows, on one or more tables, and either commit or roll back the changes atomically.
Uses for multi-statement transactions include:
Performing DML mutations on multiple tables as a single transaction. The tables can span multiple datasets or projects.
Performing mutations on a single table in several stages, based on intermediate computations.
Transactions guarantee ACID properties and support snapshot isolation. During a transaction, all reads return a consistent snapshot of the tables referenced in the transaction. If a statement in a transaction modifies a table, the changes are visible to subsequent statements within the same transaction.
In the case of only one DML statement, BigQuery is also ACID complaint. That is documented here.
If an update fails, you won't have X amount of rows updated.
please I want to understand the difference between the folowing two statements:
insert into table_name values (,,,,,);
and
insert into table_name values (,,,,,);
commit;
If you insert data without commit you can select data from database and see it. But other users can't.
It's better to look to sql documentation:
Until you commit a transaction:
You can see any changes you have made during the transaction by
querying the modified tables, but other users cannot see the changes.
After you commit the transaction, the changes are visible to other
users' statements that execute after the commit.
You can roll back (undo) any changes made during the transaction with
the ROLLBACK statement (see ROLLBACK.
for example here Oracle Documentation
and some info about transactions
All the DML (insert, update , delete) to be inserted in the database you have to commit them, like approve that you want to add them in the database. If you dont commit DML statment , it will not be enter in the database.
what is commit ?
Docs.oracle cant describe it better
Use the COMMIT statement to end your current transaction and make
permanent all changes performed in the transaction. A transaction is a
sequence of SQL statements that Oracle Database treats as a single
unit. his statement also erases all savepoints in the transaction and
releases transaction locks.
Do I need to issue commit command after running the stored function with select query?
Yes, you do, in some cases (please read the discussion in the link below). The rule is: Always commit if you made change in DB (after DML commands), even with SELECT statement.
Use the COMMIT statement to end your current transaction and make permanent all changes performed in the transaction.
Read more: oracle - what statements need to be committed?
Thanks #Ben for the head up!
commit means "save the changes"
select statement does not change any data.
changing data can be done by Insert, update, delete statements (Data Manipulation language) .
You need to commit an sql statement only if you have performed a DML statement (INSERT, DELETE, UPDATE, MERGE) in your stored procedure. So, if you only queried the data, there is no need to commit.
I know that in MySQL ddl statements such as alter table/create table/etc cause an implicit transaction commit.
As we are moving to PostgreSQL is it possible to wrap multiple DDL statments in a transaction?
This would make migration scripts a lot more robust, a failed DDL change would cause everything to rollback.
DDL statements are covered by transactions. I can't find the relevant section in the official documentation, but have provided a link to the wiki which covers it.
Just remember that transactions aren't automatically opened in postgresql, you must start them with BEGIN or START TRANSACTION.
Postgresql Wiki about Transactional DDL
Not every Postgres DDL statement can be wrapped in transaction. Statements like DROP DATABASE / DROP TABLESPACE and some other file-system-related cant rollback.
Also:
ALTER TYPE ... ADD VALUE (the form that adds a new value to an enum
type) cannot be executed inside a transaction block.
Also some statements like TRUNCATE are 'not MVCC save'. Changes, made by that kind of statements can affect other queries, even if they are rolled back.
So - read the official manual for your version of postgres to find out if your DDL's are transaction safe.
In an Oracle stored procedure, how do I write a transaction? Do I need to do it explicitly or will Oracle automatically lock rows?
You might want to browse the concept guide, in particular the chapter about transactions:
A transaction is a logical unit of work that comprises one or more SQL statements run by a single user. [...] A transaction begins with the user's first executable SQL statement. A transaction ends when it is explicitly committed or rolled back by that user.
You don't have to explicitely start a transaction, it is done automatically. You will have to specify the end of the transaction with a commit (or a rollback).
The locking mechanism is a fundamental part of the DB, read about it in the chapter Data Concurrency and Consistency.
Regarding stored procedures
A stored procedure is a set of statements, they are executed in the same transaction as the calling session (*). Usually, transaction control (commit and rollback) belongs to the calling application. The calling app has a wider vision of the process (which may involve several stored procedures) and is therefore in a better position to determine if the data is in a consistent state. While you can commit in a stored procedure, it is not the norm.
(*) except if the procedure is declared as an autonomous transaction, in which case the procedure is executed as an independent session (thanks be here now, now I see your point).
#AdamStevenson Concerning DDL, there's a cite from the Concept's Guide:
If the
current transaction contains any DML statements, Oracle first commits
the
transaction, and then runs and commits the DDL statement as a new,
single
statement transaction.
So if you have started a transaction before the DDL statement (e.g. wrote an INSERT, UPDATE, DELETE, MERGE statements), the transaction started will be implicitly commited - you should always keep that in mind when processing DML statements.
I agree with Vincent Malgrat, you might find some very useful information about transaction processing at the Concept's Guide.