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- <!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>13.5. Caveats</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="applevel-consistency.html" title="13.4. Data Consistency Checks at the Application Level" /><link rel="next" href="locking-indexes.html" title="13.6. Locking and Indexes" /></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">13.5. Caveats</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="applevel-consistency.html" title="13.4. Data Consistency Checks at the Application Level">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="mvcc.html" title="Chapter 13. Concurrency Control">Up</a></td><th width="60%" align="center">Chapter 13. Concurrency Control</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="locking-indexes.html" title="13.6. Locking and Indexes">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="MVCC-CAVEATS"><div class="titlepage"><div><div><h2 class="title" style="clear: both">13.5. Caveats</h2></div></div></div><p>
- Some DDL commands, currently only <a class="xref" href="sql-truncate.html" title="TRUNCATE"><span class="refentrytitle">TRUNCATE</span></a> and the
- table-rewriting forms of <a class="xref" href="sql-altertable.html" title="ALTER TABLE"><span class="refentrytitle">ALTER TABLE</span></a>, are not
- MVCC-safe. This means that after the truncation or rewrite commits, the
- table will appear empty to concurrent transactions, if they are using a
- snapshot taken before the DDL command committed. This will only be an
- issue for a transaction that did not access the table in question
- before the DDL command started — any transaction that has done so
- would hold at least an <code class="literal">ACCESS SHARE</code> table lock,
- which would block the DDL command until that transaction completes.
- So these commands will not cause any apparent inconsistency in the
- table contents for successive queries on the target table, but they
- could cause visible inconsistency between the contents of the target
- table and other tables in the database.
- </p><p>
- Support for the Serializable transaction isolation level has not yet
- been added to Hot Standby replication targets (described in
- <a class="xref" href="hot-standby.html" title="26.5. Hot Standby">Section 26.5</a>). The strictest isolation level currently
- supported in hot standby mode is Repeatable Read. While performing all
- permanent database writes within Serializable transactions on the
- master will ensure that all standbys will eventually reach a consistent
- state, a Repeatable Read transaction run on the standby can sometimes
- see a transient state that is inconsistent with any serial execution
- of the transactions on the master.
- </p><p>
- Internal access to the system catalogs is not done using the isolation
- level of the current transaction. This means that newly created database
- objects such as tables are visible to concurrent Repeatable Read and
- Serializable transactions, even though the rows they contain are not. In
- contrast, queries that explicitly examine the system catalogs don't see
- rows representing concurrently created database objects, in the higher
- isolation levels.
- </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="applevel-consistency.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="mvcc.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="locking-indexes.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">13.4. Data Consistency Checks at the Application Level </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 13.6. Locking and Indexes</td></tr></table></div></body></html>
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