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Green_GoodERP18_FSJ_Standard/PostgreSQL/doc/postgresql/html/fdw-row-locking.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>56.5. Row Locking in Foreign Data Wrappers</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="fdw-planning.html" title="56.4. Foreign Data Wrapper Query Planning" /><link rel="next" href="tablesample-method.html" title="Chapter 57. Writing a Table Sampling Method" /></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">56.5. Row Locking in Foreign Data Wrappers</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="fdw-planning.html" title="56.4. Foreign Data Wrapper Query Planning">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="fdwhandler.html" title="Chapter 56. Writing a Foreign Data Wrapper">Up</a></td><th width="60%" align="center">Chapter 56. Writing a Foreign Data Wrapper</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="tablesample-method.html" title="Chapter 57. Writing a Table Sampling Method">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="FDW-ROW-LOCKING"><div class="titlepage"><div><div><h2 class="title" style="clear: both">56.5. Row Locking in Foreign Data Wrappers</h2></div></div></div><p>

  3.      If an FDW's underlying storage mechanism has a concept of locking

  4.      individual rows to prevent concurrent updates of those rows, it is

  5.      usually worthwhile for the FDW to perform row-level locking with as

  6.      close an approximation as practical to the semantics used in

  7.      ordinary <span class="productname">PostgreSQL</span> tables.  There are multiple

  8.      considerations involved in this.

  9.     </p><p>

  10.      One key decision to be made is whether to perform <em class="firstterm">early

  11.      locking</em> or <em class="firstterm">late locking</em>.  In early locking, a row is

  12.      locked when it is first retrieved from the underlying store, while in

  13.      late locking, the row is locked only when it is known that it needs to

  14.      be locked.  (The difference arises because some rows may be discarded by

  15.      locally-checked restriction or join conditions.)  Early locking is much

  16.      simpler and avoids extra round trips to a remote store, but it can cause

  17.      locking of rows that need not have been locked, resulting in reduced

  18.      concurrency or even unexpected deadlocks.  Also, late locking is only

  19.      possible if the row to be locked can be uniquely re-identified later.

  20.      Preferably the row identifier should identify a specific version of the

  21.      row, as <span class="productname">PostgreSQL</span> TIDs do.

  22.     </p><p>

  23.      By default, <span class="productname">PostgreSQL</span> ignores locking considerations

  24.      when interfacing to FDWs, but an FDW can perform early locking without

  25.      any explicit support from the core code.  The API functions described

  26.      in <a class="xref" href="fdw-callbacks.html#FDW-CALLBACKS-ROW-LOCKING" title="56.2.5. FDW Routines for Row Locking">Section 56.2.5</a>, which were added

  27.      in <span class="productname">PostgreSQL</span> 9.5, allow an FDW to use late locking if

  28.      it wishes.

  29.     </p><p>

  30.      An additional consideration is that in <code class="literal">READ COMMITTED</code>

  31.      isolation mode, <span class="productname">PostgreSQL</span> may need to re-check

  32.      restriction and join conditions against an updated version of some

  33.      target tuple.  Rechecking join conditions requires re-obtaining copies

  34.      of the non-target rows that were previously joined to the target tuple.

  35.      When working with standard <span class="productname">PostgreSQL</span> tables, this is

  36.      done by including the TIDs of the non-target tables in the column list

  37.      projected through the join, and then re-fetching non-target rows when

  38.      required.  This approach keeps the join data set compact, but it

  39.      requires inexpensive re-fetch capability, as well as a TID that can

  40.      uniquely identify the row version to be re-fetched.  By default,

  41.      therefore, the approach used with foreign tables is to include a copy of

  42.      the entire row fetched from a foreign table in the column list projected

  43.      through the join.  This puts no special demands on the FDW but can

  44.      result in reduced performance of merge and hash joins.  An FDW that is

  45.      capable of meeting the re-fetch requirements can choose to do it the

  46.      first way.

  47.     </p><p>

  48.      For an <code class="command">UPDATE</code> or <code class="command">DELETE</code> on a foreign table, it

  49.      is recommended that the <code class="literal">ForeignScan</code> operation on the target

  50.      table perform early locking on the rows that it fetches, perhaps via the

  51.      equivalent of <code class="command">SELECT FOR UPDATE</code>.  An FDW can detect whether

  52.      a table is an <code class="command">UPDATE</code>/<code class="command">DELETE</code> target at plan time

  53.      by comparing its relid to <code class="literal">root-&gt;parse-&gt;resultRelation</code>,

  54.      or at execution time by using <code class="function">ExecRelationIsTargetRelation()</code>.

  55.      An alternative possibility is to perform late locking within the

  56.      <code class="function">ExecForeignUpdate</code> or <code class="function">ExecForeignDelete</code>

  57.      callback, but no special support is provided for this.

  58.     </p><p>

  59.      For foreign tables that are specified to be locked by a <code class="command">SELECT

  60.      FOR UPDATE/SHARE</code> command, the <code class="literal">ForeignScan</code> operation can

  61.      again perform early locking by fetching tuples with the equivalent

  62.      of <code class="command">SELECT FOR UPDATE/SHARE</code>.  To perform late locking

  63.      instead, provide the callback functions defined

  64.      in <a class="xref" href="fdw-callbacks.html#FDW-CALLBACKS-ROW-LOCKING" title="56.2.5. FDW Routines for Row Locking">Section 56.2.5</a>.

  65.      In <code class="function">GetForeignRowMarkType</code>, select rowmark option

  66.      <code class="literal">ROW_MARK_EXCLUSIVE</code>, <code class="literal">ROW_MARK_NOKEYEXCLUSIVE</code>,

  67.      <code class="literal">ROW_MARK_SHARE</code>, or <code class="literal">ROW_MARK_KEYSHARE</code> depending

  68.      on the requested lock strength.  (The core code will act the same

  69.      regardless of which of these four options you choose.)

  70.      Elsewhere, you can detect whether a foreign table was specified to be

  71.      locked by this type of command by using <code class="function">get_plan_rowmark</code> at

  72.      plan time, or <code class="function">ExecFindRowMark</code> at execution time; you must

  73.      check not only whether a non-null rowmark struct is returned, but that

  74.      its <code class="structfield">strength</code> field is not <code class="literal">LCS_NONE</code>.

  75.     </p><p>

  76.      Lastly, for foreign tables that are used in an <code class="command">UPDATE</code>,

  77.      <code class="command">DELETE</code> or <code class="command">SELECT FOR UPDATE/SHARE</code> command but

  78.      are not specified to be row-locked, you can override the default choice

  79.      to copy entire rows by having <code class="function">GetForeignRowMarkType</code> select

  80.      option <code class="literal">ROW_MARK_REFERENCE</code> when it sees lock strength

  81.      <code class="literal">LCS_NONE</code>.  This will cause <code class="function">RefetchForeignRow</code> to

  82.      be called with that value for <code class="structfield">markType</code>; it should then

  83.      re-fetch the row without acquiring any new lock.  (If you have

  84.      a <code class="function">GetForeignRowMarkType</code> function but don't wish to re-fetch

  85.      unlocked rows, select option <code class="literal">ROW_MARK_COPY</code>

  86.      for <code class="literal">LCS_NONE</code>.)

  87.     </p><p>

  88.      See <code class="filename">src/include/nodes/lockoptions.h</code>, the comments

  89.      for <code class="type">RowMarkType</code> and <code class="type">PlanRowMark</code>

  90.      in <code class="filename">src/include/nodes/plannodes.h</code>, and the comments for

  91.      <code class="type">ExecRowMark</code> in <code class="filename">src/include/nodes/execnodes.h</code> for

  92.      additional information.

  93.     </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="fdw-planning.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="fdwhandler.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="tablesample-method.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">56.4. Foreign Data Wrapper Query Planning </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 57. Writing a Table Sampling Method</td></tr></table></div></body></html>
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