среда, 1 августа 2012 г.

db: mysql


DROP TABLE automatically commits the current active transaction, unless you use the TEMPORARY keyword.

Postgres - http://www.postgresql.org/docs/devel/static/mvcc.html

1) MVCC == Multiversion Concurrency Control
2) The main advantage of using the MVCC model of concurrency control rather than locking is that in MVCC locks acquired for querying (reading) data do not conflict with locks acquired for writing data, and so reading never blocks writing and writing never blocks reading. PostgreSQL maintains this guarantee even when providing the strictest level of transaction isolation through the use of an innovative Serializable Snapshot Isolation (SSI) level.
3) Table- and row-level locking facilities are also available in PostgreSQL for applications which don't generally need full transaction isolation and prefer to explicitly manage particular points of conflict.

Table 13-1. Standard SQL Transaction Isolation Levels
Isolation LevelDirty ReadNonrepeatable ReadPhantom Read
Read uncommittedPossiblePossiblePossible
Read committedNot possiblePossiblePossible
Repeatable readNot possibleNot possiblePossible
SerializableNot possibleNot possibleNot possible
In PostgreSQL, you can request any of the four standard transaction isolation levels
SELECT query (without a FOR UPDATE/SHARE clause)