Add support for REFRESH MATERIALIZED VIEW CONCURRENTLY.

This allows reads to continue without any blocking while a REFRESH
runs.  The new data appears atomically as part of transaction
commit.

Review questioned the Assert that a matview was not a system
relation.  This will be addressed separately.

Reviewed by Hitoshi Harada, Robert Haas, Andres Freund.
Merged after review with security patch f3ab5d4.

2 files changed, 34 insertions, 3 deletions

diff --git a/doc/src/sgml/mvcc.sgml b/doc/src/sgml/mvcc.sgml
index 316add70b70..cefd3235a6b 100644
--- a/doc/src/sgml/mvcc.sgml
+++ b/doc/src/sgml/mvcc.sgml
@@ -928,8 +928,7 @@ ERROR:  could not serialize access due to read/write dependencies among transact
         </para>
 
         <para>
-         This lock mode is not automatically acquired on tables by any
-         <productname>PostgreSQL</productname> command.
+         Acquired by <command>REFRESH MATERIALIZED VIEW CONCURRENTLY</command>.
         </para>
        </listitem>
       </varlistentry>
diff --git a/doc/src/sgml/ref/refresh_materialized_view.sgml b/doc/src/sgml/ref/refresh_materialized_view.sgml
index 8f59bbf1231..d2f8104aa7d 100644
--- a/doc/src/sgml/ref/refresh_materialized_view.sgml
+++ b/doc/src/sgml/ref/refresh_materialized_view.sgml
@@ -21,7 +21,7 @@ PostgreSQL documentation
 
  <refsynopsisdiv>
 <synopsis>
-REFRESH MATERIALIZED VIEW <replaceable class="PARAMETER">name</replaceable>
+REFRESH MATERIALIZED VIEW [ CONCURRENTLY ] <replaceable class="PARAMETER">name</replaceable>
     [ WITH [ NO ] DATA ]
 </synopsis>
  </refsynopsisdiv>
@@ -38,6 +38,10 @@ REFRESH MATERIALIZED VIEW <replaceable class="PARAMETER">name</replaceable>
    data is generated and the materialized view is left in an unscannable
    state.
   </para>
+  <para>
+   <literal>CONCURRENTLY</literal> and <literal>WITH NO DATA</literal> may not
+   be specified together.
+  </para>
  </refsect1>
 
  <refsect1>
@@ -45,6 +49,34 @@ REFRESH MATERIALIZED VIEW <replaceable class="PARAMETER">name</replaceable>
 
   <variablelist>
    <varlistentry>
+    <term><literal>CONCURRENTLY</literal></term>
+    <listitem>
+     <para>
+      Refresh the materialized view without locking out concurrent selects on
+      the materialized view.  Without this option a refresh which affects a
+      lot of rows will tend to use fewer resources and complete more quickly,
+      but could block other connections which are trying to read from the
+      materialized view.  This option may be faster in cases where a small
+      number of rows are affected.
+     </para>
+     <para>
+      This option is only allowed if there is at least one
+      <literal>UNIQUE</literal> index on the materialized view which uses only
+      column names and includes all rows; that is, it must not index on any
+      expressions nor include a <literal>WHERE</literal> clause.
+     </para>
+     <para>
+      This option may not be used when the materialized view is not already
+      populated.
+     </para>
+     <para>
+      Even with this option only one <literal>REFRESH</literal> at a time may
+      run against any one materialized view.
+     </para>
+    </listitem>
+   </varlistentry>
+
+   <varlistentry>
     <term><replaceable class="PARAMETER">name</replaceable></term>
     <listitem>
      <para>