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Commit 3f86238f authored by Thomas G. Lockhart's avatar Thomas G. Lockhart
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Make minor changes in wording. 

Adjust tags to get a clean build.
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doc/src/sgml/ref/lock.sgml
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− 87
View file @ 3f86238f
......@@ -53,13 +53,13 @@ LOCK [ TABLE ] <replaceable class="PARAMETER">table</replaceable> IN SHARE ROW E
<para>
This lock mode is acquired automatically over tables being queried.
<productname>Postgres</productname> releases automatically acquired
ACCESS SHARE locks after statement is done.
ACCESS SHARE locks after the statement is done.
</para>
</note>
<para>
This is the less restrictive lock mode which conflicts with
ACCESS EXCLUSIVE mode only. It's intended to protect table being
This is the least restrictive lock mode which conflicts only with
ACCESS EXCLUSIVE mode. It is intended to protect a table being
queried from concurrent <command>ALTER TABLE</command>,
<command>DROP TABLE</command> and <command>VACUUM</command>
statements over the same table.
......@@ -74,7 +74,7 @@ LOCK [ TABLE ] <replaceable class="PARAMETER">table</replaceable> IN SHARE ROW E
<listitem>
<note>
<para>
Automatically acquired by <command>SELECT FOR UPDATE</command> statement.
Automatically acquired by any <command>SELECT FOR UPDATE</command> statement.
</para>
</note>
......@@ -91,15 +91,15 @@ LOCK [ TABLE ] <replaceable class="PARAMETER">table</replaceable> IN SHARE ROW E
<listitem>
<note>
<para>
Automatically acquired by <command>UPDATE</command>,
<command>DELETE</command>, <command>INSERT</command> statements.
Automatically acquired by any <command>UPDATE</command>,
<command>DELETE</command>, <command>INSERT</command> statement.
</para>
</note>
<para>
Conflicts with SHARE, SHARE ROW EXCLUSIVE, EXCLUSIVE and
ACCESS EXCLUSIVE modes. Generally means that a transaction
updated/inserted some tuples in a table.
updated or inserted some tuples in a table.
</para>
</listitem>
</varlistentry>
......@@ -111,7 +111,7 @@ LOCK [ TABLE ] <replaceable class="PARAMETER">table</replaceable> IN SHARE ROW E
<listitem>
<note>
<para>
Automatically acquired by <command>CREATE INDEX</command> statement.
Automatically acquired by any <command>CREATE INDEX</command> statement.
</para>
</note>
......@@ -147,7 +147,7 @@ LOCK [ TABLE ] <replaceable class="PARAMETER">table</replaceable> IN SHARE ROW E
<para>
Conflicts with ROW SHARE, ROW EXCLUSIVE, SHARE, SHARE ROW EXCLUSIVE,
EXCLUSIVE and ACCESS EXCLUSIVE modes. This mode is yet more
restrictive than SHARE ROW EXCLUSIVE one - it blocks concurrent
restrictive than that of SHARE ROW EXCLUSIVE; it blocks all concurrent
SELECT FOR UPDATE queries.
</para>
</listitem>
......@@ -167,13 +167,13 @@ LOCK [ TABLE ] <replaceable class="PARAMETER">table</replaceable> IN SHARE ROW E
<para>
This is the most restrictive lock mode which conflicts with all other
lock modes and protects locked table from any concurrent operations.
lock modes and protects a locked table from any concurrent operations.
</para>
<note>
<para>
This lock mode is also acquired by first form of
<command>LOCK TABLE</command> (i.e. without explicit
This lock mode is also acquired by an unqualified
<command>LOCK TABLE</command> (i.e. the command without an explicit
lock mode option).
</para>
</note>
......@@ -218,41 +218,48 @@ LOCK [ TABLE ] <replaceable class="PARAMETER">table</replaceable> IN SHARE ROW E
Description
</title>
<para>
<productname>Postgres</productname> always uses less restrictive
lock modes ever possible. <command>LOCK TABLE</command> statement
provided for cases when you might need in more restrictive locking.
<productname>Postgres</productname> always uses the least restrictive
lock mode whenever possible. <command>LOCK TABLE</command>
provided for cases when you might need more restrictive locking.
</para>
<para>
For example, application run transaction at READ COMMITTED isolation
level and need to ensure existance data in a table for duration of
transaction. To achieve this you could use SHARE lock mode over
For example, an application runs a transaction at READ COMMITTED isolation
level and needs to ensure the existance of data in a table for the
duration of the
transaction. To achieve this you could use SHARE lock mode over the
table before querying. This will protect data from concurrent changes
and provide your further read operations over table with data in their
real current state, because of SHARE lock mode conflicts with ROW EXCLUSIVE
one, acquired by writers, and your LOCK TABLE table IN SHARE MODE statement
will wait untill concurrent write operations (if any) commit/rollback.
(Note that to read data in their real current state running transaction
at SERIALIZABLE isolation level you have to execute LOCK TABLE
statement before execution any DML statement, when transaction defines
what concurrent changes will be visible to herself).
and provide any further read operations over the table with data in their
actual current state, because SHARE lock mode conflicts with any ROW EXCLUSIVE
one acquired by writers, and your LOCK TABLE table IN SHARE MODE statement
will wait until any concurrent write operations commit or rollback.
<note>
<para>
To read data in their real current state when running a transaction
at the SERIALIZABLE isolation level you have to execute a LOCK TABLE
statement before execution any DML statement, when the transaction defines
what concurrent changes will be visible to itself.
</para>
</note>
</para>
<para>
If, in addition to requirements above, transaction is going to
In addition to the requirements above, if a transaction is going to
change data in a table then SHARE ROW EXCLUSIVE lock mode should
be acquired to prevent deadlock conditions when two concurrent
transactions would lock table in SHARE mode and than would
transactions attempt to lock the table in SHARE mode and then
try to change data in this table, both (implicitly) acquiring
ROW EXCLUSIVE lock mode that conflicts with concurrent SHARE lock.
</para>
<para>
Following deadlock issue (when two transaction wait one another)
touched above, you should follow two general rules to prevent
To continue with the deadlock (when two transaction wait one another)
issue raised above, you should follow two general rules to prevent
deadlock conditions:
</para>
<itemizedlist>
<listitem>
<para>
Transactions have to acquire locks on the same objects in the same order.
......@@ -260,9 +267,9 @@ LOCK [ TABLE ] <replaceable class="PARAMETER">table</replaceable> IN SHARE ROW E
<para>
For example, if one application updates row R1 and than updates
row R2 (in the same transaction) then second application shouldn't
update row R2 if it's going update row R1 later (in single transaction).
Instead, it should update R1 and R2 rows in the same order as first
row R2 (in the same transaction) then the second application shouldn't
update row R2 if it's going to update row R1 later (in a single transaction).
Instead, it should update rows R1 and R2 in the same order as the first
application.
</para>
</listitem>
......@@ -271,39 +278,44 @@ LOCK [ TABLE ] <replaceable class="PARAMETER">table</replaceable> IN SHARE ROW E
<para>
Transactions should acquire two conflicting lock modes only if
one of them is self-conflicting (i.e. may be held by one
transaction at time only) and should acquire most restrictive
mode first.
transaction at time only). If multiple lock modes are involved,
then transactions should always acquire the most restrictive mode first.
</para>
<para>
Example for this rule is described above when told about using
SHARE ROW EXCLUSIVE mode instead of SHARE one.
An example for this rule was given previously when discussing the
use of SHARE ROW EXCLUSIVE mode rather than SHARE mode.
</para>
</listitem>
</itemizedlist>
<note>
<para>
<productname>Postgres</productname> does detect deadlocks and will
rollback one of waiting transactions to resolve the deadlock.
rollback at least one waiting transaction to resolve the deadlock.
</para>
</note>
<refsect2 id="R2-SQL-LOCK-3">
<refsect2info>
<date>1998-09-24</date>
<date>1999-06-08</date>
</refsect2info>
<title>
Notes
</title>
<para>
<command>LOCK</command> is a <productname>Postgres</productname>
language extension.
</para>
<para>
Except for ACCESS SHARE/EXCLUSIVE lock modes, all other
<productname>Postgres</productname> lock modes and
<command>LOCK TABLE</command> syntax are compatible with
<productname>Oracle</productname> ones.
<productname>Postgres</productname> lock modes and the
<command>LOCK TABLE</command> syntax are compatible with those
present in <productname>Oracle</productname>.
</para>
<para>
<command>LOCK</command> works only inside transactions.
</para>
......@@ -329,7 +341,7 @@ LOCK [ TABLE ] <replaceable class="PARAMETER">table</replaceable> IN SHARE ROW E
-- Do ROLLBACK if record was not returned
--
INSERT INTO films_user_comments VALUES
(_id_, 'GREAT! I was waiting it so long!');
(_id_, 'GREAT! I was waiting for it for so long!');
COMMIT WORK;
</programlisting>
</para>
......@@ -366,7 +378,8 @@ LOCK [ TABLE ] <replaceable class="PARAMETER">table</replaceable> IN SHARE ROW E
<para>
There is no <command>LOCK TABLE</command> in <acronym>SQL92</acronym>,
which instead uses <command>SET TRANSACTION</command> to specify
concurrency level on transactions. We support that too.
concurrency level on transactions. We support that too; see
<xref linkend="SQL-SET-TITLE" endterm="SQL-SET-TITLE"> for details.
</para>
</refsect2>
</refsect1>
......
doc/src/sgml/ref/set.sgml
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− 1
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......@@ -6,7 +6,7 @@
<refmiscinfo>SQL - Language Statements</refmiscinfo>
</refmeta>
<refnamediv>
<refname>
<refname id="SQL-SET-TITLE">
SET
</refname>
<refpurpose>
......
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