Reduce lock levels of some ALTER TABLE cmds

VALIDATE CONSTRAINT

CLUSTER ON
SET WITHOUT CLUSTER

ALTER COLUMN SET STATISTICS
ALTER COLUMN SET ()
ALTER COLUMN RESET ()

All other sub-commands use AccessExclusiveLock

Simon Riggs and Noah Misch

Reviews by Robert Haas and Andres Freund

doc/src/sgml/mvcc.sgml

@@ -865,7 +865,9 @@ 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>CREATE INDEX CONCURRENTLY</>, and
-         some forms of <command>ALTER TABLE</command>.
+         <command>ALTER TABLE VALIDATE</command> and other
+         <command>ALTER TABLE</command> variants (for full details see
+         <xref linkend="SQL-ALTERTABLE">).
         </para>
        </listitem>
       </varlistentry>
@@ -951,10 +953,11 @@ ERROR:  could not serialize access due to read/write dependencies among transact
         </para>
 
         <para>
-         Acquired by the <command>ALTER TABLE</>, <command>DROP TABLE</>,
+         Acquired by the <command>DROP TABLE</>,
          <command>TRUNCATE</command>, <command>REINDEX</command>,
          <command>CLUSTER</command>, and <command>VACUUM FULL</command>
-         commands.
+         commands. Many forms of <command>ALTER TABLE</> also acquire
+         a lock at this level (see <xref linkend="SQL-ALTERTABLE">).
          This is also the default lock mode for <command>LOCK TABLE</command>
          statements that do not specify a mode explicitly.
         </para>

doc/src/sgml/ref/alter_table.sgml

@@ -84,7 +84,10 @@ ALTER TABLE [ IF EXISTS ] <replaceable class="PARAMETER">name</replaceable>
 
   <para>
    <command>ALTER TABLE</command> changes the definition of an existing table.
-   There are several subforms:
+   There are several subforms described below. Note that the lock level required
+   may differ for each subform. An <literal>ACCESS EXCLUSIVE</literal> lock is held
+   unless explicitly noted. When multiple subcommands are listed, the lock
+   held will be the strictest one required from any subcommand.
 
   <variablelist>
    <varlistentry>
@@ -181,6 +184,9 @@ ALTER TABLE [ IF EXISTS ] <replaceable class="PARAMETER">name</replaceable>
       <productname>PostgreSQL</productname> query planner, refer to
       <xref linkend="planner-stats">.
      </para>
+     <para>
+      SET STATISTICS acquires a <literal>SHARE UPDATE EXCLUSIVE</literal> lock.
+     </para>
     </listitem>
    </varlistentry>
 
@@ -213,6 +219,10 @@ ALTER TABLE [ IF EXISTS ] <replaceable class="PARAMETER">name</replaceable>
       of statistics by the <productname>PostgreSQL</productname> query
       planner, refer to <xref linkend="planner-stats">.
      </para>
+     <para>
+      Changing per-attribute options acquires a
+      <literal>SHARE UPDATE EXCLUSIVE</literal> lock.
+     </para>
     </listitem>
    </varlistentry>
 
@@ -338,11 +348,17 @@ ALTER TABLE [ IF EXISTS ] <replaceable class="PARAMETER">name</replaceable>
       Nothing happens if the constraint is already marked valid.
      </para>
      <para>
-      Validation can be a long process on larger tables and currently requires
-      an <literal>ACCESS EXCLUSIVE</literal> lock.  The value of separating
+      Validation can be a long process on larger tables. The value of separating
       validation from initial creation is that you can defer validation to less
       busy times, or can be used to give additional time to correct pre-existing
-      errors while preventing new errors.
+      errors while preventing new errors. Note also that validation on its own
+      does not prevent normal write commands against the table while it runs.
+     </para>
+     <para>
+      Validation acquires only a <literal>SHARE UPDATE EXCLUSIVE</literal> lock
+      on the table being altered. If the constraint is a foreign key then
+      a <literal>ROW SHARE</literal> lock is also required on
+      the table referenced by the constraint.
      </para>
     </listitem>
    </varlistentry>
@@ -408,6 +424,9 @@ ALTER TABLE [ IF EXISTS ] <replaceable class="PARAMETER">name</replaceable>
       <xref linkend="SQL-CLUSTER">
       operations.  It does not actually re-cluster the table.
      </para>
+     <para>
+      Changing cluster options acquires a <literal>SHARE UPDATE EXCLUSIVE</literal> lock.
+     </para>
     </listitem>
    </varlistentry>
 
@@ -420,6 +439,9 @@ ALTER TABLE [ IF EXISTS ] <replaceable class="PARAMETER">name</replaceable>
       index specification from the table.  This affects
       future cluster operations that don't specify an index.
      </para>
+     <para>
+      Changing cluster options acquires a <literal>SHARE UPDATE EXCLUSIVE</literal> lock.
+     </para>
     </listitem>
    </varlistentry>
 
@@ -1078,6 +1100,14 @@ ALTER TABLE distributors ADD CONSTRAINT distfk FOREIGN KEY (address) REFERENCES
 </programlisting>
   </para>
 
+  <para>
+   To add a foreign key constraint to a table with the least impact on other work:
+<programlisting>
+ALTER TABLE distributors ADD CONSTRAINT distfk FOREIGN KEY (address) REFERENCES addresses (address) NOT VALID;
+ALTER TABLE distributors VALIDATE CONSTRAINT distfk;
+</programlisting>
+  </para>
+
   <para>
    To add a (multicolumn) unique constraint to a table:
 <programlisting>