Linux SRV-16 6.18.44-paas #1 SMP PREEMPT_DYNAMIC Thu Aug 13 10:08:12 UTC 2026 x86_64
/opt/postgresql-9.4.19-201809281555/doc/html/
//opt/postgresql-9.4.19-201809281555/doc/html/logicaldecoding-output-plugin.html
 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<HTML
><HEAD
><TITLE
>Logical Decoding Output Plugins</TITLE
><META
NAME="GENERATOR"
CONTENT="Modular DocBook HTML Stylesheet Version 1.79"><LINK
REV="MADE"
HREF="mailto:pgsql-docs@postgresql.org"><LINK
REL="HOME"
TITLE="PostgreSQL 9.4.19 Documentation"
HREF="index.html"><LINK
REL="UP"
TITLE="Logical Decoding"
HREF="logicaldecoding.html"><LINK
REL="PREVIOUS"
TITLE="System Catalogs Related to Logical Decoding"
HREF="logicaldecoding-catalogs.html"><LINK
REL="NEXT"
TITLE="Logical Decoding Output Writers"
HREF="logicaldecoding-writer.html"><LINK
REL="STYLESHEET"
TYPE="text/css"
HREF="stylesheet.css"><META
HTTP-EQUIV="Content-Type"
CONTENT="text/html; charset=ISO-8859-1"><META
NAME="creation"
CONTENT="2018-08-06T20:33:28"></HEAD
><BODY
CLASS="SECT1"
><DIV
CLASS="NAVHEADER"
><TABLE
SUMMARY="Header navigation table"
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TH
COLSPAN="4"
ALIGN="center"
VALIGN="bottom"
><A
HREF="index.html"
>PostgreSQL 9.4.19 Documentation</A
></TH
></TR
><TR
><TD
WIDTH="10%"
ALIGN="left"
VALIGN="top"
><A
TITLE="System Catalogs Related to Logical Decoding"
HREF="logicaldecoding-catalogs.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="10%"
ALIGN="left"
VALIGN="top"
><A
HREF="logicaldecoding.html"
ACCESSKEY="U"
>Up</A
></TD
><TD
WIDTH="60%"
ALIGN="center"
VALIGN="bottom"
>Chapter 46. Logical Decoding</TD
><TD
WIDTH="20%"
ALIGN="right"
VALIGN="top"
><A
TITLE="Logical Decoding Output Writers"
HREF="logicaldecoding-writer.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
></TABLE
><HR
ALIGN="LEFT"
WIDTH="100%"></DIV
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="LOGICALDECODING-OUTPUT-PLUGIN"
>46.6. Logical Decoding Output Plugins</A
></H1
><P
>    An example output plugin can be found in the
    <A
HREF="test-decoding.html"
>     <TT
CLASS="FILENAME"
>contrib/test_decoding</TT
>
    </A
>
    subdirectory of the PostgreSQL source tree.
   </P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="LOGICALDECODING-OUTPUT-INIT"
>46.6.1. Initialization Function</A
></H2
><P
>     An output plugin is loaded by dynamically loading a shared library with
     the output plugin's name as the library base name. The normal library
     search path is used to locate the library. To provide the required output
     plugin callbacks and to indicate that the library is actually an output
     plugin it needs to provide a function named
     <CODE
CLASS="FUNCTION"
>_PG_output_plugin_init</CODE
>. This function is passed a
     struct that needs to be filled with the callback function pointers for
     individual actions.
</P><PRE
CLASS="PROGRAMLISTING"
>typedef struct OutputPluginCallbacks
{
    LogicalDecodeStartupCB startup_cb;
    LogicalDecodeBeginCB begin_cb;
    LogicalDecodeChangeCB change_cb;
    LogicalDecodeCommitCB commit_cb;
    LogicalDecodeShutdownCB shutdown_cb;
} OutputPluginCallbacks;

typedef void (*LogicalOutputPluginInit) (struct OutputPluginCallbacks *cb);</PRE
><P>
     The <CODE
CLASS="FUNCTION"
>begin_cb</CODE
>, <CODE
CLASS="FUNCTION"
>change_cb</CODE
>
     and <CODE
CLASS="FUNCTION"
>commit_cb</CODE
> callbacks are required,
     while <CODE
CLASS="FUNCTION"
>startup_cb</CODE
>
     and <CODE
CLASS="FUNCTION"
>shutdown_cb</CODE
> are optional.
    </P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="LOGICALDECODING-CAPABILITIES"
>46.6.2. Capabilities</A
></H2
><P
>     To decode, format and output changes, output plugins can use most of the
     backend's normal infrastructure, including calling output functions. Read
     only access to relations is permitted as long as only relations are
     accessed that either have been created by <TT
CLASS="COMMAND"
>initdb</TT
> in
     the <TT
CLASS="LITERAL"
>pg_catalog</TT
> schema, or have been marked as user
     provided catalog tables using
</P><PRE
CLASS="PROGRAMLISTING"
>ALTER TABLE user_catalog_table SET (user_catalog_table = true);
CREATE TABLE another_catalog_table(data text) WITH (user_catalog_table = true);</PRE
><P>
     Any actions leading to transaction ID assignment are prohibited. That, among others,
     includes writing to tables, performing DDL changes, and
     calling <TT
CLASS="LITERAL"
>txid_current()</TT
>.
    </P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="LOGICALDECODING-OUTPUT-MODE"
>46.6.3. Output Modes</A
></H2
><P
>     Output plugin callbacks can pass data to the consumer in nearly arbitrary
     formats. For some use cases, like viewing the changes via SQL, returning
     data in a data type that can contain arbitrary data (e.g., <TT
CLASS="TYPE"
>bytea</TT
>) is
     cumbersome. If the output plugin only outputs textual data in the
     server's encoding, it can declare that by
     setting <TT
CLASS="LITERAL"
>OutputPluginOptions.output_type</TT
>
     to <TT
CLASS="LITERAL"
>OUTPUT_PLUGIN_TEXTUAL_OUTPUT</TT
> instead
     of <TT
CLASS="LITERAL"
>OUTPUT_PLUGIN_BINARY_OUTPUT</TT
> in
     the <A
HREF="logicaldecoding-output-plugin.html#LOGICALDECODING-OUTPUT-PLUGIN-STARTUP"
>startup
     callback</A
>. In that case, all the data has to be in the server's encoding
     so that a <TT
CLASS="TYPE"
>text</TT
> datum can contain it. This is checked in assertion-enabled
     builds.
    </P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="LOGICALDECODING-OUTPUT-PLUGIN-CALLBACKS"
>46.6.4. Output Plugin Callbacks</A
></H2
><P
>     An output plugin gets notified about changes that are happening via
     various callbacks it needs to provide.
    </P
><P
>     Concurrent transactions are decoded in commit order, and only changes
     belonging to a specific transaction are decoded between
     the <TT
CLASS="LITERAL"
>begin</TT
> and <TT
CLASS="LITERAL"
>commit</TT
>
     callbacks. Transactions that were rolled back explicitly or implicitly
     never get
     decoded. Successful savepoints are
     folded into the transaction containing them in the order they were
     executed within that transaction.
    </P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>      Only transactions that have already safely been flushed to disk will be
      decoded. That can lead to a <TT
CLASS="COMMAND"
>COMMIT</TT
> not immediately being decoded in a
      directly following <TT
CLASS="LITERAL"
>pg_logical_slot_get_changes()</TT
>
      when <TT
CLASS="VARNAME"
>synchronous_commit</TT
> is set
      to <TT
CLASS="LITERAL"
>off</TT
>.
     </P
></BLOCKQUOTE
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="LOGICALDECODING-OUTPUT-PLUGIN-STARTUP"
>46.6.4.1. Startup Callback</A
></H3
><P
>      The optional <CODE
CLASS="FUNCTION"
>startup_cb</CODE
> callback is called whenever
      a replication slot is created or asked to stream changes, independent
      of the number of changes that are ready to be put out.
</P><PRE
CLASS="PROGRAMLISTING"
>typedef void (*LogicalDecodeStartupCB) (struct LogicalDecodingContext *ctx,
                                        OutputPluginOptions *options,
                                        bool is_init);</PRE
><P>
      The <TT
CLASS="LITERAL"
>is_init</TT
> parameter will be true when the
      replication slot is being created and false
      otherwise. <TT
CLASS="PARAMETER"
>options</TT
> points to a struct of options
      that output plugins can set:
</P><PRE
CLASS="PROGRAMLISTING"
>typedef struct OutputPluginOptions
{
    OutputPluginOutputType output_type;
} OutputPluginOptions;</PRE
><P>
      <TT
CLASS="LITERAL"
>output_type</TT
> has to either be set to
      <TT
CLASS="LITERAL"
>OUTPUT_PLUGIN_TEXTUAL_OUTPUT</TT
>
      or <TT
CLASS="LITERAL"
>OUTPUT_PLUGIN_BINARY_OUTPUT</TT
>. See also
      <A
HREF="logicaldecoding-output-plugin.html#LOGICALDECODING-OUTPUT-MODE"
>Section 46.6.3</A
>.
     </P
><P
>      The startup callback should validate the options present in
      <TT
CLASS="LITERAL"
>ctx-&gt;output_plugin_options</TT
>. If the output plugin
      needs to have a state, it can
      use <TT
CLASS="LITERAL"
>ctx-&gt;output_plugin_private</TT
> to store it.
     </P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="LOGICALDECODING-OUTPUT-PLUGIN-SHUTDOWN"
>46.6.4.2. Shutdown Callback</A
></H3
><P
>      The optional <CODE
CLASS="FUNCTION"
>shutdown_cb</CODE
> callback is called
      whenever a formerly active replication slot is not used anymore and can
      be used to deallocate resources private to the output plugin. The slot
      isn't necessarily being dropped, streaming is just being stopped.
</P><PRE
CLASS="PROGRAMLISTING"
>typedef void (*LogicalDecodeShutdownCB) (struct LogicalDecodingContext *ctx);</PRE
><P>
     </P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="LOGICALDECODING-OUTPUT-PLUGIN-BEGIN"
>46.6.4.3. Transaction Begin Callback</A
></H3
><P
>      The required <CODE
CLASS="FUNCTION"
>begin_cb</CODE
> callback is called whenever a
      start of a committed transaction has been decoded. Aborted transactions
      and their contents never get decoded.
</P><PRE
CLASS="PROGRAMLISTING"
>typedef void (*LogicalDecodeBeginCB) (struct LogicalDecodingContext *ctx,
                                      ReorderBufferTXN *txn);</PRE
><P>
      The <TT
CLASS="PARAMETER"
>txn</TT
> parameter contains meta information about
      the transaction, like the time stamp at which it has been committed and
      its XID.
     </P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="LOGICALDECODING-OUTPUT-PLUGIN-COMMIT"
>46.6.4.4. Transaction End Callback</A
></H3
><P
>      The required <CODE
CLASS="FUNCTION"
>commit_cb</CODE
> callback is called whenever
      a transaction commit has been
      decoded. The <CODE
CLASS="FUNCTION"
>change_cb</CODE
> callbacks for all modified
      rows will have been called before this, if there have been any modified
      rows.
</P><PRE
CLASS="PROGRAMLISTING"
>typedef void (*LogicalDecodeCommitCB) (struct LogicalDecodingContext *ctx,
                                       ReorderBufferTXN *txn,
                                       XLogRecPtr commit_lsn);</PRE
><P>
     </P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="LOGICALDECODING-OUTPUT-PLUGIN-CHANGE"
>46.6.4.5. Change Callback</A
></H3
><P
>      The required <CODE
CLASS="FUNCTION"
>change_cb</CODE
> callback is called for every
      individual row modification inside a transaction, may it be
      an <TT
CLASS="COMMAND"
>INSERT</TT
>, <TT
CLASS="COMMAND"
>UPDATE</TT
>,
      or <TT
CLASS="COMMAND"
>DELETE</TT
>. Even if the original command modified
      several rows at once the callback will be called individually for each
      row.
</P><PRE
CLASS="PROGRAMLISTING"
>typedef void (*LogicalDecodeChangeCB) (struct LogicalDecodingContext *ctx,
                                       ReorderBufferTXN *txn,
                                       Relation relation,
                                       ReorderBufferChange *change);</PRE
><P>
      The <TT
CLASS="PARAMETER"
>ctx</TT
> and <TT
CLASS="PARAMETER"
>txn</TT
> parameters
      have the same contents as for the <CODE
CLASS="FUNCTION"
>begin_cb</CODE
>
      and <CODE
CLASS="FUNCTION"
>commit_cb</CODE
> callbacks, but additionally the
      relation descriptor <TT
CLASS="PARAMETER"
>relation</TT
> points to the
      relation the row belongs to and a struct
      <TT
CLASS="PARAMETER"
>change</TT
> describing the row modification are passed
      in.
     </P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>       Only changes in user defined tables that are not unlogged
       (see <A
HREF="sql-createtable.html#SQL-CREATETABLE-UNLOGGED"
><I
CLASS="TERM"
><TT
CLASS="LITERAL"
>UNLOGGED</TT
></I
></A
>) and not temporary
       (see <A
HREF="sql-createtable.html#SQL-CREATETABLE-TEMPORARY"
><I
CLASS="TERM"
><TT
CLASS="LITERAL"
>TEMPORARY</TT
> or <TT
CLASS="LITERAL"
>TEMP</TT
></I
></A
>) can be extracted using
       logical decoding.
      </P
></BLOCKQUOTE
></DIV
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="LOGICALDECODING-OUTPUT-PLUGIN-OUTPUT"
>46.6.5. Functions for Producing Output</A
></H2
><P
>     To actually produce output, output plugins can write data to
     the <TT
CLASS="LITERAL"
>StringInfo</TT
> output buffer
     in <TT
CLASS="LITERAL"
>ctx-&gt;out</TT
> when inside
     the <CODE
CLASS="FUNCTION"
>begin_cb</CODE
>, <CODE
CLASS="FUNCTION"
>commit_cb</CODE
>,
     or <CODE
CLASS="FUNCTION"
>change_cb</CODE
> callbacks. Before writing to the output
     buffer, <CODE
CLASS="FUNCTION"
>OutputPluginPrepareWrite(ctx, last_write)</CODE
> has
     to be called, and after finishing writing to the
     buffer, <CODE
CLASS="FUNCTION"
>OutputPluginWrite(ctx, last_write)</CODE
> has to be
     called to perform the write. The <TT
CLASS="PARAMETER"
>last_write</TT
>
     indicates whether a particular write was the callback's last write.
    </P
><P
>     The following example shows how to output data to the consumer of an
     output plugin:
</P><PRE
CLASS="PROGRAMLISTING"
>OutputPluginPrepareWrite(ctx, true);
appendStringInfo(ctx-&#62;out, "BEGIN %u", txn-&#62;xid);
OutputPluginWrite(ctx, true);</PRE
><P>
    </P
></DIV
></DIV
><DIV
CLASS="NAVFOOTER"
><HR
ALIGN="LEFT"
WIDTH="100%"><TABLE
SUMMARY="Footer navigation table"
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
><A
HREF="logicaldecoding-catalogs.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="index.html"
ACCESSKEY="H"
>Home</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
><A
HREF="logicaldecoding-writer.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
>System Catalogs Related to Logical Decoding</TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="logicaldecoding.html"
ACCESSKEY="U"
>Up</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
>Logical Decoding Output Writers</TD
></TR
></TABLE
></DIV
></BODY
></HTML
>