REINDEX CONCURRENTLY

This adds the CONCURRENTLY option to the REINDEX command.  A REINDEX
CONCURRENTLY on a specific index creates a new index (like CREATE
INDEX CONCURRENTLY), then renames the old index away and the new index
in place and adjusts the dependencies, and then drops the old
index (like DROP INDEX CONCURRENTLY).  The REINDEX command also has
the capability to run its other variants (TABLE, DATABASE) with the
CONCURRENTLY option (but not SYSTEM).

The reindexdb command gets the --concurrently option.

Author: Michael Paquier, Andreas Karlsson, Peter Eisentraut
Reviewed-by: Andres Freund, Fujii Masao, Jim Nasby, Sergei Kornilov
Discussion: https://www.postgresql.org/message-id/flat/60052986-956b-4478-45ed-8bd119e9b9cf%402ndquadrant.com#74948a1044c56c5e817a5050f554ddee

4 files changed, 200 insertions, 2 deletions

diff --git a/doc/src/sgml/mvcc.sgml b/doc/src/sgml/mvcc.sgml
index bedd9a008d3..9b7ef8bf095 100644
--- a/doc/src/sgml/mvcc.sgml
+++ b/doc/src/sgml/mvcc.sgml
@@ -926,6 +926,7 @@ ERROR:  could not serialize access due to read/write dependencies among transact
         <para>
          Acquired by <command>VACUUM</command> (without <option>FULL</option>),
          <command>ANALYZE</command>, <command>CREATE INDEX CONCURRENTLY</command>,
+         <command>REINDEX CONCURRENTLY</command>,
          <command>CREATE STATISTICS</command>, and certain <command>ALTER
          INDEX</command> and <command>ALTER TABLE</command> variants (for full
          details see <xref linkend="sql-alterindex"/> and <xref
diff --git a/doc/src/sgml/ref/create_index.sgml b/doc/src/sgml/ref/create_index.sgml
index d8f018f4dac..d9d95b20e36 100644
--- a/doc/src/sgml/ref/create_index.sgml
+++ b/doc/src/sgml/ref/create_index.sgml
@@ -844,6 +844,7 @@ CREATE INDEX CONCURRENTLY sales_quantity_index ON sales_table (quantity);
   <simplelist type="inline">
    <member><xref linkend="sql-alterindex"/></member>
    <member><xref linkend="sql-dropindex"/></member>
+   <member><xref linkend="sql-reindex"/></member>
   </simplelist>
  </refsect1>
 </refentry>
diff --git a/doc/src/sgml/ref/reindex.sgml b/doc/src/sgml/ref/reindex.sgml
index 47cef987d48..ccabb330cbf 100644
--- a/doc/src/sgml/ref/reindex.sgml
+++ b/doc/src/sgml/ref/reindex.sgml
@@ -21,7 +21,7 @@ PostgreSQL documentation
 
  <refsynopsisdiv>
 <synopsis>
-REINDEX [ ( VERBOSE ) ] { INDEX | TABLE | SCHEMA | DATABASE | SYSTEM } <replaceable class="parameter">name</replaceable>
+REINDEX [ ( VERBOSE ) ] { INDEX | TABLE | SCHEMA | DATABASE | SYSTEM } [ CONCURRENTLY ] <replaceable class="parameter">name</replaceable>
 </synopsis>
  </refsynopsisdiv>
 
@@ -68,7 +68,7 @@ REINDEX [ ( VERBOSE ) ] { INDEX | TABLE | SCHEMA | DATABASE | SYSTEM } <replacea
       An index build with the <literal>CONCURRENTLY</literal> option failed, leaving
       an <quote>invalid</quote> index. Such indexes are useless but it can be
       convenient to use <command>REINDEX</command> to rebuild them. Note that
-      <command>REINDEX</command> will not perform a concurrent build. To build the
+      <command>REINDEX</command> will not perform a concurrent build on an invalid index. To build the
       index without interfering with production you should drop the index and
       reissue the <command>CREATE INDEX CONCURRENTLY</command> command.
      </para>
@@ -152,6 +152,21 @@ REINDEX [ ( VERBOSE ) ] { INDEX | TABLE | SCHEMA | DATABASE | SYSTEM } <replacea
    </varlistentry>
 
    <varlistentry>
+    <term><literal>CONCURRENTLY</literal></term>
+    <listitem>
+     <para>
+      When this option is used, <productname>PostgreSQL</productname> will rebuild the
+      index without taking any locks that prevent concurrent inserts,
+      updates, or deletes on the table; whereas a standard reindex build
+      locks out writes (but not reads) on the table until it's done.
+      There are several caveats to be aware of when using this option
+      &mdash; see <xref linkend="sql-reindex-concurrently"
+      endterm="sql-reindex-concurrently-title"/>.
+     </para>
+    </listitem>
+   </varlistentry>
+
+   <varlistentry>
     <term><literal>VERBOSE</literal></term>
     <listitem>
      <para>
@@ -241,6 +256,159 @@ REINDEX [ ( VERBOSE ) ] { INDEX | TABLE | SCHEMA | DATABASE | SYSTEM } <replacea
    Each individual partition can be reindexed separately instead.
   </para>
 
+  <refsect2 id="sql-reindex-concurrently">
+   <title id="sql-reindex-concurrently-title">Rebuilding Indexes Concurrently</title>
+
+   <indexterm zone="sql-reindex-concurrently">
+    <primary>index</primary>
+    <secondary>rebuilding concurrently</secondary>
+   </indexterm>
+
+   <para>
+    Rebuilding an index can interfere with regular operation of a database.
+    Normally <productname>PostgreSQL</productname> locks the table whose index is rebuilt
+    against writes and performs the entire index build with a single scan of the
+    table. Other transactions can still read the table, but if they try to
+    insert, update, or delete rows in the table they will block until the
+    index rebuild is finished. This could have a severe effect if the system is
+    a live production database. Very large tables can take many hours to be
+    indexed, and even for smaller tables, an index rebuild can lock out writers
+    for periods that are unacceptably long for a production system.
+   </para>
+
+   <para>
+    <productname>PostgreSQL</productname> supports rebuilding indexes with minimum locking
+    of writes.  This method is invoked by specifying the
+    <literal>CONCURRENTLY</literal> option of <command>REINDEX</command>. When this option
+    is used, <productname>PostgreSQL</productname> must perform two scans of the table
+    for each index that needs to be rebuild and in addition it must wait for
+    all existing transactions that could potentially use the index to
+    terminate. This method requires more total work than a standard index
+    rebuild and takes significantly longer to complete as it needs to wait
+    for unfinished transactions that might modify the index. However, since
+    it allows normal operations to continue while the index is rebuilt, this
+    method is useful for rebuilding indexes in a production environment. Of
+    course, the extra CPU, memory and I/O load imposed by the index rebuild
+    may slow down other operations.
+   </para>
+
+   <para>
+    The following steps occur in a concurrent reindex.  Each step is run in a
+    separate transaction.  If there are multiple indexes to be rebuilt, then
+    each step loops through all the indexes before moving to the next step.
+
+    <orderedlist>
+     <listitem>
+      <para>
+       A new temporary index definition is added into the catalog
+       <literal>pg_index</literal>.  This definition will be used to replace
+       the old index.  A <literal>SHARE UPDATE EXCLUSIVE</literal> lock at
+       session level is taken on the indexes being reindexed as well as its
+       associated table to prevent any schema modification while processing.
+      </para>
+     </listitem>
+
+     <listitem>
+      <para>
+       A first pass to build the index is done for each new index.  Once the
+       index is built, its flag <literal>pg_index.indisready</literal> is
+       switched to <quote>true</quote> to make ready for inserts, making it
+       visible to other sessions once the transaction that performed the build
+       is finished.  This step is done in a separate transaction for each
+       index.
+      </para>
+     </listitem>
+
+     <listitem>
+      <para>
+       Then a second pass is performed to add tuples that were added while the
+       first pass build was running.  This step is also done in a separate
+       transaction for each index.
+      </para>
+     </listitem>
+
+     <listitem>
+      <para>
+       All the constraints that refer to the index are changed to refer to the
+       new index definition, and the names of the indexes are changed.  At
+       this point <literal>pg_index.indisvalid</literal> is switched to
+       <quote>true</quote> for the new index and to <quote>false</quote> for
+       the old, and a cache invalidation is done so as all the sessions that
+       referenced the old index are invalidated.
+      </para>
+     </listitem>
+
+     <listitem>
+      <para>
+       The old indexes have <literal>pg_index.indisready</literal> switched to
+       <quote>false</quote> to prevent any new tuple insertions, after waiting
+       for running queries that might reference the old index to complete.
+      </para>
+     </listitem>
+
+     <listitem>
+      <para>
+       The old indexes are dropped.  The <literal>SHARE UPDATE
+       EXCLUSIVE</literal> session locks for the indexes and the table ar
+       released.
+      </para>
+     </listitem>
+    </orderedlist>
+   </para>
+
+   <para>
+    If a problem arises while rebuilding the indexes, such as a
+    uniqueness violation in a unique index, the <command>REINDEX</command>
+    command will fail but leave behind an <quote>invalid</quote> new index on top
+    of the existing one. This index will be ignored for querying purposes
+    because it might be incomplete; however it will still consume update
+    overhead. The <application>psql</application> <command>\d</command> command will report
+    such an index as <literal>INVALID</literal>:
+
+<programlisting>
+postgres=# \d tab
+       Table "public.tab"
+ Column |  Type   | Modifiers
+--------+---------+-----------
+ col    | integer |
+Indexes:
+    "idx" btree (col)
+    "idx_ccnew" btree (col) INVALID
+</programlisting>
+
+    The recommended recovery method in such cases is to drop the invalid index
+    and try again to perform <command>REINDEX CONCURRENTLY</command>.  The
+    concurrent index created during the processing has a name ending in the
+    suffix <literal>ccnew</literal>, or <literal>ccold</literal> if it is an
+    old index definition which we failed to drop. Invalid indexes can be
+    dropped using <literal>DROP INDEX</literal>, including invalid toast
+    indexes.
+   </para>
+
+   <para>
+    Regular index builds permit other regular index builds on the same table
+    to occur in parallel, but only one concurrent index build can occur on a
+    table at a time. In both cases, no other types of schema modification on
+    the table are allowed meanwhile.  Another difference is that a regular
+    <command>REINDEX TABLE</command> or <command>REINDEX INDEX</command>
+    command can be performed within a transaction block, but <command>REINDEX
+    CONCURRENTLY</command> cannot.
+   </para>
+
+   <para>
+    <command>REINDEX SYSTEM</command> does not support
+    <command>CONCURRENTLY</command> since system catalogs cannot be reindexed
+    concurrently.
+   </para>
+
+   <para>
+    Furthermore, indexes for exclusion constraints cannot be reindexed
+    concurrently.  If such an index is named directly in this command, an
+    error is raised.  If a table or database with exclusion constraint indexes
+    is reindexed concurrently, those indexes will be skipped.  (It is possible
+    to reindex such indexes without the concurrently option.)
+   </para>
+  </refsect2>
  </refsect1>
 
  <refsect1>
@@ -273,6 +441,14 @@ $ <userinput>psql broken_db</userinput>
 broken_db=&gt; REINDEX DATABASE broken_db;
 broken_db=&gt; \q
 </programlisting></para>
+
+  <para>
+   Rebuild a table while authorizing read and write operations on involved
+   relations when performed:
+
+<programlisting>
+REINDEX TABLE CONCURRENTLY my_broken_table;
+</programlisting></para>
  </refsect1>
 
  <refsect1>
@@ -282,4 +458,14 @@ broken_db=&gt; \q
    There is no <command>REINDEX</command> command in the SQL standard.
   </para>
  </refsect1>
+
+ <refsect1>
+  <title>See Also</title>
+
+  <simplelist type="inline">
+   <member><xref linkend="sql-createindex"/></member>
+   <member><xref linkend="sql-dropindex"/></member>
+   <member><xref linkend="app-reindexdb"/></member>
+  </simplelist>
+ </refsect1>
 </refentry>
diff --git a/doc/src/sgml/ref/reindexdb.sgml b/doc/src/sgml/ref/reindexdb.sgml
index 1273dad8072..cdfac3fe4f9 100644
--- a/doc/src/sgml/ref/reindexdb.sgml
+++ b/doc/src/sgml/ref/reindexdb.sgml
@@ -119,6 +119,16 @@ PostgreSQL documentation
      </varlistentry>
 
      <varlistentry>
+      <term><option>--concurrently</option></term>
+      <listitem>
+       <para>
+        Use the <literal>CONCURRENTLY</literal> option.  See <xref
+        linkend="sql-reindex"/> for further information.
+       </para>
+      </listitem>
+     </varlistentry>
+
+     <varlistentry>
       <term><option><optional>-d</optional> <replaceable class="parameter">dbname</replaceable></option></term>
       <term><option><optional>--dbname=</optional><replaceable class="parameter">dbname</replaceable></option></term>
       <listitem>