Sophie: postgresql-docs-7.4.1-2.5.100mdk i586

postgresql-docs-7.4.1-2.5.100mdk.i586.rpm

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>PostgreSQL 7.4.1 Documentation</TH
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>Chapter 12. Concurrency Control</H1
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><B
>Table of Contents</B
></DT
><DT
>12.1. <A
HREF="mvcc.html#MVCC-INTRO"
>Introduction</A
></DT
><DT
>12.2. <A
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>Transaction Isolation</A
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><DD
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>12.2.1. <A
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>Read Committed Isolation Level</A
></DT
><DT
>12.2.2. <A
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>Serializable Isolation Level</A
></DT
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>12.3. <A
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>Explicit Locking</A
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>12.3.1. <A
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>Table-Level Locks</A
></DT
><DT
>12.3.2. <A
HREF="explicit-locking.html#LOCKING-ROWS"
>Row-Level Locks</A
></DT
><DT
>12.3.3. <A
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>Deadlocks</A
></DT
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>12.4. <A
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>Data Consistency Checks at the Application Level</A
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>12.5. <A
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>Locking and Indexes</A
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><P
>   This chapter describes the behavior of the
   <SPAN
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>PostgreSQL</SPAN
> database system when two or
   more sessions try to access the same data at the same time.  The
   goals in that situation are to allow efficient access for all
   sessions while maintaining strict data integrity.  Every developer
   of database applications should be familiar with the topics covered
   in this chapter.
  </P
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><A
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>12.1. Introduction</A
></H1
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><P
>    Unlike traditional database systems which use locks for concurrency control,
    <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
    maintains data consistency by using a multiversion model
    (Multiversion Concurrency Control, <ACRONYM
CLASS="ACRONYM"
>MVCC</ACRONYM
>). 
    This means that while querying a database each transaction sees
    a snapshot of data (a <I
CLASS="FIRSTTERM"
>database version</I
>)
    as it was some
    time ago, regardless of the current state of the underlying data.
    This protects the transaction from viewing inconsistent data that
    could be caused by (other) concurrent transaction updates on the same
    data rows, providing <I
CLASS="FIRSTTERM"
>transaction isolation</I
>
    for each database session.
   </P
><P
>    The main advantage to using the <ACRONYM
CLASS="ACRONYM"
>MVCC</ACRONYM
> model of
    concurrency control rather than locking is that in
    <ACRONYM
CLASS="ACRONYM"
>MVCC</ACRONYM
> 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.
   </P
><P
>    Table- and row-level locking facilities are also available in
    <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> for applications that cannot
    adapt easily to <ACRONYM
CLASS="ACRONYM"
>MVCC</ACRONYM
> behavior.  However, proper
    use of <ACRONYM
CLASS="ACRONYM"
>MVCC</ACRONYM
> will generally provide better
    performance than locks.
   </P
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