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Commit1d174c3

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Explain privileges required for LOCK. Minor wordsmithing too.
1 parent9ad7379 commit1d174c3

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‎doc/src/sgml/ref/lock.sgml

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@@ -1,5 +1,5 @@
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<!--
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$Header: /cvsroot/pgsql/doc/src/sgml/ref/lock.sgml,v 1.29 2001/12/08 03:24:37 thomas Exp $
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$Header: /cvsroot/pgsql/doc/src/sgml/ref/lock.sgml,v 1.30 2002/01/18 01:05:43 tgl Exp $
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PostgreSQL documentation
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-->
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@@ -76,8 +76,7 @@ where <replaceable class="PARAMETER">lockmode</replaceable> is one of:
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<listitem>
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<note>
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<para>
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Automatically acquired by <command>SELECT...FOR UPDATE</command>.
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While it is a shared lock, may be upgraded later to a ROW EXCLUSIVE lock.
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Automatically acquired by <command>SELECT ... FOR UPDATE</command>.
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</para>
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</note>
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@@ -175,6 +174,9 @@ where <replaceable class="PARAMETER">lockmode</replaceable> is one of:
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Conflicts with ROW SHARE, ROW EXCLUSIVE, SHARE UPDATE EXCLUSIVE,
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SHARE, SHARE ROW EXCLUSIVE,
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EXCLUSIVE and ACCESS EXCLUSIVE modes.
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This mode allows only concurrent ACCESS SHARE, i.e., only reads
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from the table can proceed in parallel with a transaction holding
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this lock mode.
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</para>
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</listitem>
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</varlistentry>
@@ -225,7 +227,7 @@ LOCK TABLE
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</computeroutput></term>
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<listitem>
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<para>
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The lock was successfullyapplied.
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The lock was successfullyacquired.
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</para>
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</listitem>
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</varlistentry>
@@ -310,23 +312,26 @@ ERROR <replaceable class="PARAMETER">name</replaceable>: Table does not exist.
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</para>
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<para>
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For example, an application runs a transaction at READ COMMITTED isolation
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level and needs to ensure the existence of data in a table for the
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duration of the
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transaction. To achieve this you coulduse SHARE lock mode over the
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table before querying. This willprotect data fromconcurrent changes
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andprovide anyfurther read operations over the tablewith data in their
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actual current state, because SHARE lock mode conflicts with any ROW EXCLUSIVE
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one acquired by writers, and your
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For example,supposean application runs a transaction at READ COMMITTED
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isolationlevel and needs to ensure the existence of data in a table for
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theduration of the
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transaction. To achieve this you couldobtain SHARE lock mode over the
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table before querying. This willpreventconcurrent data changes
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andensurefurther read operations over the tablesee data in their
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actual current state, because SHARE lock mode conflicts with any ROW
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EXCLUSIVE lock acquired by writers, and your
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<command>LOCK TABLE <replaceable class="PARAMETER">name</replaceable> IN SHARE MODE</command>
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statement will wait until any concurrent write operations commit or rollback.
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statement will wait until any concurrent write operations commit or
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rollback. Thus, once you obtain the lock, there are no uncommitted
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writes outstanding.
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<note>
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<para>
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To read data in their real current state when running a transaction
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at the SERIALIZABLE isolation level you have to execute a LOCK TABLE
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statement before executing any DML statement, when the transaction defines
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what concurrent changes will be visible to itself.
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To read data in their actual current state when running a transaction
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at the SERIALIZABLE isolation level, you have to execute the LOCK TABLE
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statement before executing any DML statement. A serializable
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transaction's view of data will be frozen when its first DML statement
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begins.
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</para>
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</note>
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</para>
@@ -341,7 +346,7 @@ ERROR <replaceable class="PARAMETER">name</replaceable>: Table does not exist.
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</para>
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<para>
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To continue with the deadlock (when twotransaction wait for one another)
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To continue with the deadlock (when twotransactions wait for one another)
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issue raised above, you should follow two general rules to prevent
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deadlock conditions:
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</para>
@@ -364,8 +369,8 @@ ERROR <replaceable class="PARAMETER">name</replaceable>: Table does not exist.
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<listitem>
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<para>
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Transactions should acquire two conflicting lock modes only if
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one of them is self-conflicting (i.e., may be held by one
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transaction attime only). If multiple lock modes are involved,
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one of them is self-conflicting (i.e., may be held byonlyone
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transaction ata time). If multiple lock modes are involved,
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then transactions should always acquire the most restrictive mode first.
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</para>
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@@ -399,19 +404,17 @@ ERROR <replaceable class="PARAMETER">name</replaceable>: Table does not exist.
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</title>
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<para>
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<command>LOCK</command> is a <productname>PostgreSQL</productname>
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language extension.
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<literal>LOCK ... IN ACCESS SHARE MODE</> requires <literal>SELECT</>
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privileges on the target table. All other forms of <command>LOCK</>
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require <literal>UPDATE</> and/or <literal>DELETE</> privileges.
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</para>
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<para>
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Except for ACCESS SHARE, ACCESS EXCLUSIVE, and SHARE UPDATE EXCLUSIVE lock
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modes, the <productname>PostgreSQL</productname> lock modes and the
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<command>LOCK TABLE</command> syntax are compatible with those
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present in <productname>Oracle</productname>.
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</para>
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<para>
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<command>LOCK</command> works only inside transactions.
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<command>LOCK</command> is useful only inside a transaction block
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(<command>BEGIN</>...<command>COMMIT</>), since the lock is dropped
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as soon as the transaction ends. A <command>LOCK</> command appearing
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outside any transaction block forms a self-contained transaction, so the
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lock will be dropped as soon as it is obtained.
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</para>
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</refsect2>
@@ -471,6 +474,13 @@ COMMIT WORK;
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concurrency levels on transactions. We support that too; see
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<xref linkend="SQL-SET-TRANSACTION" endterm="SQL-SET-TRANSACTION-TITLE"> for details.
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</para>
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<para>
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Except for ACCESS SHARE, ACCESS EXCLUSIVE, and SHARE UPDATE EXCLUSIVE lock
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modes, the <productname>PostgreSQL</productname> lock modes and the
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<command>LOCK TABLE</command> syntax are compatible with those
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present in <productname>Oracle</productname>(TM).
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</para>
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</refsect2>
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</refsect1>
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</refentry>

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