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Green_GoodERP18_FSJ_Standard/PostgreSQL/doc/postgresql/html/sql-set-transaction.html

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  1. <?xml version="1.0" encoding="UTF-8" standalone="no"?>

  2. <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>SET TRANSACTION</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets V1.79.1" /><link rel="prev" href="sql-set-session-authorization.html" title="SET SESSION AUTHORIZATION" /><link rel="next" href="sql-show.html" title="SHOW" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">SET TRANSACTION</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="sql-set-session-authorization.html" title="SET SESSION AUTHORIZATION">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="sql-commands.html" title="SQL Commands">Up</a></td><th width="60%" align="center">SQL Commands</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 12.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="sql-show.html" title="SHOW">Next</a></td></tr></table><hr></hr></div><div class="refentry" id="SQL-SET-TRANSACTION"><div class="titlepage"></div><a id="id-1.9.3.177.1" class="indexterm"></a><a id="id-1.9.3.177.2" class="indexterm"></a><a id="id-1.9.3.177.3" class="indexterm"></a><a id="id-1.9.3.177.4" class="indexterm"></a><div class="refnamediv"><h2><span class="refentrytitle">SET TRANSACTION</span></h2><p>SET TRANSACTION — set the characteristics of the current transaction</p></div><div class="refsynopsisdiv"><h2>Synopsis</h2><pre class="synopsis">

  3. SET TRANSACTION <em class="replaceable"><code>transaction_mode</code></em> [, ...]

  4. SET TRANSACTION SNAPSHOT <em class="replaceable"><code>snapshot_id</code></em>

  5. SET SESSION CHARACTERISTICS AS TRANSACTION <em class="replaceable"><code>transaction_mode</code></em> [, ...]

  6. 

  7. <span class="phrase">where <em class="replaceable"><code>transaction_mode</code></em> is one of:</span>

  8. 

  9.     ISOLATION LEVEL { SERIALIZABLE | REPEATABLE READ | READ COMMITTED | READ UNCOMMITTED }

  10.     READ WRITE | READ ONLY

  11.     [ NOT ] DEFERRABLE

  12. </pre></div><div class="refsect1" id="id-1.9.3.177.8"><h2>Description</h2><p>

  13.    The <code class="command">SET TRANSACTION</code> command sets the

  14.    characteristics of the current transaction. It has no effect on any

  15.    subsequent transactions.  <code class="command">SET SESSION

  16.    CHARACTERISTICS</code> sets the default transaction

  17.    characteristics for subsequent transactions of a session.  These

  18.    defaults can be overridden by <code class="command">SET TRANSACTION</code>

  19.    for an individual transaction.

  20.   </p><p>

  21.    The available transaction characteristics are the transaction

  22.    isolation level, the transaction access mode (read/write or

  23.    read-only), and the deferrable mode.

  24.    In addition, a snapshot can be selected, though only for the current

  25.    transaction, not as a session default.

  26.   </p><p>

  27.    The isolation level of a transaction determines what data the

  28.    transaction can see when other transactions are running concurrently:

  29. 

  30.    </p><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="literal">READ COMMITTED</code></span></dt><dd><p>

  31.        A statement can only see rows committed before it began. This

  32.        is the default.

  33.       </p></dd><dt><span class="term"><code class="literal">REPEATABLE READ</code></span></dt><dd><p>

  34.        All statements of the current transaction can only see rows committed

  35.        before the first query or data-modification statement was executed in

  36.        this transaction.

  37.       </p></dd><dt><span class="term"><code class="literal">SERIALIZABLE</code></span></dt><dd><p>

  38.        All statements of the current transaction can only see rows committed

  39.        before the first query or data-modification statement was executed in

  40.        this transaction.  If a pattern of reads and writes among concurrent

  41.        serializable transactions would create a situation which could not

  42.        have occurred for any serial (one-at-a-time) execution of those

  43.        transactions, one of them will be rolled back with a

  44.        <code class="literal">serialization_failure</code> error.

  45.       </p></dd></dl></div><p>

  46. 

  47.    The SQL standard defines one additional level, <code class="literal">READ

  48.    UNCOMMITTED</code>.

  49.    In <span class="productname">PostgreSQL</span> <code class="literal">READ

  50.    UNCOMMITTED</code> is treated as <code class="literal">READ COMMITTED</code>.

  51.   </p><p>

  52.    The transaction isolation level cannot be changed after the first query or

  53.    data-modification statement (<code class="command">SELECT</code>,

  54.    <code class="command">INSERT</code>, <code class="command">DELETE</code>,

  55.    <code class="command">UPDATE</code>, <code class="command">FETCH</code>, or

  56.    <code class="command">COPY</code>) of a transaction has been executed.  See

  57.    <a class="xref" href="mvcc.html" title="Chapter 13. Concurrency Control">Chapter 13</a> for more information about transaction

  58.    isolation and concurrency control.

  59.   </p><p>

  60.    The transaction access mode determines whether the transaction is

  61.    read/write or read-only.  Read/write is the default.  When a

  62.    transaction is read-only, the following SQL commands are

  63.    disallowed: <code class="literal">INSERT</code>, <code class="literal">UPDATE</code>,

  64.    <code class="literal">DELETE</code>, and <code class="literal">COPY FROM</code> if the

  65.    table they would write to is not a temporary table; all

  66.    <code class="literal">CREATE</code>, <code class="literal">ALTER</code>, and

  67.    <code class="literal">DROP</code> commands; <code class="literal">COMMENT</code>,

  68.    <code class="literal">GRANT</code>, <code class="literal">REVOKE</code>,

  69.    <code class="literal">TRUNCATE</code>; and <code class="literal">EXPLAIN ANALYZE</code>

  70.    and <code class="literal">EXECUTE</code> if the command they would execute is

  71.    among those listed.  This is a high-level notion of read-only that

  72.    does not prevent all writes to disk.

  73.   </p><p>

  74.    The <code class="literal">DEFERRABLE</code> transaction property has no effect

  75.    unless the transaction is also <code class="literal">SERIALIZABLE</code> and

  76.    <code class="literal">READ ONLY</code>.  When all three of these properties are

  77.    selected for a

  78.    transaction, the transaction may block when first acquiring its snapshot,

  79.    after which it is able to run without the normal overhead of a

  80.    <code class="literal">SERIALIZABLE</code> transaction and without any risk of

  81.    contributing to or being canceled by a serialization failure.  This mode

  82.    is well suited for long-running reports or backups.

  83.   </p><p>

  84.    The <code class="literal">SET TRANSACTION SNAPSHOT</code> command allows a new

  85.    transaction to run with the same <em class="firstterm">snapshot</em> as an existing

  86.    transaction.  The pre-existing transaction must have exported its snapshot

  87.    with the <code class="literal">pg_export_snapshot</code> function (see <a class="xref" href="functions-admin.html#FUNCTIONS-SNAPSHOT-SYNCHRONIZATION" title="9.26.5. Snapshot Synchronization Functions">Section 9.26.5</a>).  That function returns a

  88.    snapshot identifier, which must be given to <code class="literal">SET TRANSACTION

  89.    SNAPSHOT</code> to specify which snapshot is to be imported.  The

  90.    identifier must be written as a string literal in this command, for example

  91.    <code class="literal">'000003A1-1'</code>.

  92.    <code class="literal">SET TRANSACTION SNAPSHOT</code> can only be executed at the

  93.    start of a transaction, before the first query or

  94.    data-modification statement (<code class="command">SELECT</code>,

  95.    <code class="command">INSERT</code>, <code class="command">DELETE</code>,

  96.    <code class="command">UPDATE</code>, <code class="command">FETCH</code>, or

  97.    <code class="command">COPY</code>) of the transaction.  Furthermore, the transaction

  98.    must already be set to <code class="literal">SERIALIZABLE</code> or

  99.    <code class="literal">REPEATABLE READ</code> isolation level (otherwise, the snapshot

  100.    would be discarded immediately, since <code class="literal">READ COMMITTED</code> mode takes

  101.    a new snapshot for each command).  If the importing transaction uses

  102.    <code class="literal">SERIALIZABLE</code> isolation level, then the transaction that

  103.    exported the snapshot must also use that isolation level.  Also, a

  104.    non-read-only serializable transaction cannot import a snapshot from a

  105.    read-only transaction.

  106.   </p></div><div class="refsect1" id="id-1.9.3.177.9"><h2>Notes</h2><p>

  107.    If <code class="command">SET TRANSACTION</code> is executed without a prior

  108.    <code class="command">START TRANSACTION</code> or <code class="command">BEGIN</code>,

  109.    it emits a warning and otherwise has no effect.

  110.   </p><p>

  111.    It is possible to dispense with <code class="command">SET TRANSACTION</code>

  112.    by instead specifying the desired <em class="replaceable"><code>transaction_modes</code></em> in

  113.    <code class="command">BEGIN</code> or <code class="command">START TRANSACTION</code>.

  114.    But that option is not available for <code class="command">SET TRANSACTION

  115.    SNAPSHOT</code>.

  116.   </p><p>

  117.    The session default transaction modes can also be set by setting the

  118.    configuration parameters <a class="xref" href="runtime-config-client.html#GUC-DEFAULT-TRANSACTION-ISOLATION">default_transaction_isolation</a>,

  119.    <a class="xref" href="runtime-config-client.html#GUC-DEFAULT-TRANSACTION-READ-ONLY">default_transaction_read_only</a>, and

  120.    <a class="xref" href="runtime-config-client.html#GUC-DEFAULT-TRANSACTION-DEFERRABLE">default_transaction_deferrable</a>.

  121.    (In fact <code class="command">SET SESSION CHARACTERISTICS</code> is just a

  122.    verbose equivalent for setting these variables with <code class="command">SET</code>.)

  123.    This means the defaults can be set in the configuration file, via

  124.    <code class="command">ALTER DATABASE</code>, etc.  Consult <a class="xref" href="runtime-config.html" title="Chapter 19. Server Configuration">Chapter 19</a>

  125.    for more information.

  126.   </p></div><div class="refsect1" id="id-1.9.3.177.10"><h2>Examples</h2><p>

  127.    To begin a new transaction with the same snapshot as an already

  128.    existing transaction, first export the snapshot from the existing

  129.    transaction. That will return the snapshot identifier, for example:

  130. 

  131. </p><pre class="programlisting">

  132. BEGIN TRANSACTION ISOLATION LEVEL REPEATABLE READ;

  133. SELECT pg_export_snapshot();

  134.  pg_export_snapshot

  135. ---------------------

  136.  00000003-0000001B-1

  137. (1 row)

  138. </pre><p>

  139. 

  140.    Then give the snapshot identifier in a <code class="command">SET TRANSACTION

  141.    SNAPSHOT</code> command at the beginning of the newly opened

  142.    transaction:

  143. 

  144. </p><pre class="programlisting">

  145. BEGIN TRANSACTION ISOLATION LEVEL REPEATABLE READ;

  146. SET TRANSACTION SNAPSHOT '00000003-0000001B-1';

  147. </pre></div><div class="refsect1" id="R1-SQL-SET-TRANSACTION-3"><h2>Compatibility</h2><p>

  148.    These commands are defined in the <acronym class="acronym">SQL</acronym> standard,

  149.    except for the <code class="literal">DEFERRABLE</code> transaction mode

  150.    and the <code class="command">SET TRANSACTION SNAPSHOT</code> form, which are

  151.    <span class="productname">PostgreSQL</span> extensions.

  152.   </p><p>

  153.    <code class="literal">SERIALIZABLE</code> is the default transaction

  154.    isolation level in the standard.  In

  155.    <span class="productname">PostgreSQL</span> the default is ordinarily

  156.    <code class="literal">READ COMMITTED</code>, but you can change it as

  157.    mentioned above.

  158.   </p><p>

  159.    In the SQL standard, there is one other transaction characteristic

  160.    that can be set with these commands: the size of the diagnostics

  161.    area.  This concept is specific to embedded SQL, and therefore is

  162.    not implemented in the <span class="productname">PostgreSQL</span> server.

  163.   </p><p>

  164.    The SQL standard requires commas between successive <em class="replaceable"><code>transaction_modes</code></em>, but for historical

  165.    reasons <span class="productname">PostgreSQL</span> allows the commas to be

  166.    omitted.

  167.   </p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="sql-set-session-authorization.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="sql-commands.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="sql-show.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">SET SESSION AUTHORIZATION </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> SHOW</td></tr></table></div></body></html>
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