diff options
Diffstat (limited to '')
| -rw-r--r-- | doc/src/sgml/html/transaction-id.html | 51 |
1 files changed, 51 insertions, 0 deletions
diff --git a/doc/src/sgml/html/transaction-id.html b/doc/src/sgml/html/transaction-id.html new file mode 100644 index 0000000..be51d86 --- /dev/null +++ b/doc/src/sgml/html/transaction-id.html @@ -0,0 +1,51 @@ +<?xml version="1.0" encoding="UTF-8" standalone="no"?> +<!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>74.1. Transactions and Identifiers</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 Vsnapshot" /><link rel="prev" href="transactions.html" title="Chapter 74. Transaction Processing" /><link rel="next" href="xact-locking.html" title="74.2. Transactions and Locking" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">74.1. Transactions and Identifiers</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="transactions.html" title="Chapter 74. Transaction Processing">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="transactions.html" title="Chapter 74. Transaction Processing">Up</a></td><th width="60%" align="center">Chapter 74. Transaction Processing</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 16.2 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="xact-locking.html" title="74.2. Transactions and Locking">Next</a></td></tr></table><hr /></div><div class="sect1" id="TRANSACTION-ID"><div class="titlepage"><div><div><h2 class="title" style="clear: both">74.1. Transactions and Identifiers <a href="#TRANSACTION-ID" class="id_link">#</a></h2></div></div></div><p> + Transactions can be created explicitly using <code class="command">BEGIN</code> + or <code class="command">START TRANSACTION</code> and ended using + <code class="command">COMMIT</code> or <code class="command">ROLLBACK</code>. SQL + statements outside of explicit transactions automatically use + single-statement transactions. + </p><p> + Every transaction is identified by a unique + <code class="literal">VirtualTransactionId</code> (also called + <code class="literal">virtualXID</code> or <code class="literal">vxid</code>), which + is comprised of a backend ID (or <code class="literal">backendID</code>) + and a sequentially-assigned number local to each backend, known as + <code class="literal">localXID</code>. For example, the virtual transaction + ID <code class="literal">4/12532</code> has a <code class="literal">backendID</code> + of <code class="literal">4</code> and a <code class="literal">localXID</code> of + <code class="literal">12532</code>. + </p><p> + Non-virtual <code class="literal">TransactionId</code>s (or <code class="type">xid</code>), + e.g., <code class="literal">278394</code>, are assigned sequentially to + transactions from a global counter used by all databases within + the <span class="productname">PostgreSQL</span> cluster. This assignment + happens when a transaction first writes to the database. This means + lower-numbered xids started writing before higher-numbered xids. + Note that the order in which transactions perform their first database + write might be different from the order in which the transactions + started, particularly if the transaction started with statements that + only performed database reads. + </p><p> + The internal transaction ID type <code class="type">xid</code> is 32 bits wide + and <a class="link" href="routine-vacuuming.html#VACUUM-FOR-WRAPAROUND" title="25.1.5. Preventing Transaction ID Wraparound Failures">wraps around</a> every + 4 billion transactions. A 32-bit epoch is incremented during each + wraparound. There is also a 64-bit type <code class="type">xid8</code> which + includes this epoch and therefore does not wrap around during the + life of an installation; it can be converted to xid by casting. + The functions in <a class="xref" href="functions-info.html#FUNCTIONS-PG-SNAPSHOT" title="Table 9.80. Transaction ID and Snapshot Information Functions">Table 9.80</a> + return <code class="type">xid8</code> values. Xids are used as the + basis for <span class="productname">PostgreSQL</span>'s <a class="link" href="mvcc.html" title="Chapter 13. Concurrency Control">MVCC</a> concurrency mechanism and streaming + replication. + </p><p> + When a top-level transaction with a (non-virtual) xid commits, + it is marked as committed in the <code class="filename">pg_xact</code> + directory. Additional information is recorded in the + <code class="filename">pg_commit_ts</code> directory if <a class="xref" href="runtime-config-replication.html#GUC-TRACK-COMMIT-TIMESTAMP">track_commit_timestamp</a> is enabled. + </p><p> + In addition to <code class="literal">vxid</code> and <code class="type">xid</code>, + prepared transactions are also assigned Global Transaction + Identifiers (<acronym class="acronym">GID</acronym>). GIDs are string literals up + to 200 bytes long, which must be unique amongst other currently + prepared transactions. The mapping of GID to xid is shown in <a class="link" href="view-pg-prepared-xacts.html" title="54.16. pg_prepared_xacts"><code class="structname">pg_prepared_xacts</code></a>. + </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="transactions.html" title="Chapter 74. Transaction Processing">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="transactions.html" title="Chapter 74. Transaction Processing">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="xact-locking.html" title="74.2. Transactions and Locking">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 74. Transaction Processing </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 16.2 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 74.2. Transactions and Locking</td></tr></table></div></body></html>
\ No newline at end of file |