Lower lock level for renaming indexes

Change lock level for renaming index (either ALTER INDEX or implicitly
via some other commands) from AccessExclusiveLock to
ShareUpdateExclusiveLock.

One reason we need a strong lock for relation renaming is that the
name change causes a rebuild of the relcache entry.  Concurrent
sessions that have the relation open might not be able to handle the
relcache entry changing underneath them.  Therefore, we need to lock
the relation in a way that no one can have the relation open
concurrently.  But for indexes, the relcache handles reloads specially
in RelationReloadIndexInfo() in a way that keeps changes in the
relcache entry to a minimum.  As long as no one keeps pointers to
rd_amcache and rd_options around across possible relcache flushes,
which is the case, this ought to be safe.

We also want to use a self-exclusive lock for correctness, so that
concurrent DDL doesn't overwrite the rename if they start updating
while still seeing the old version.  Therefore, we use
ShareUpdateExclusiveLock, which is already used by other DDL commands
that want to operate in a concurrent manner.

The reason this is interesting at all is that renaming an index is a
typical part of a concurrent reindexing workflow (CREATE INDEX
CONCURRENTLY new + DROP INDEX CONCURRENTLY old + rename back).  And
indeed a future built-in REINDEX CONCURRENTLY might rely on the ability
to do concurrent renames as well.

Reviewed-by: Andrey Klychkov <aaklychkov@mail.ru>
Reviewed-by: Fabrízio de Royes Mello <fabriziomello@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/1531767486.432607658@f357.i.mail.ru

2 files changed, 14 insertions, 7 deletions

diff --git a/doc/src/sgml/mvcc.sgml b/doc/src/sgml/mvcc.sgml
index 73934e5cf37..bedd9a008d3 100644
--- a/doc/src/sgml/mvcc.sgml
+++ b/doc/src/sgml/mvcc.sgml
@@ -926,10 +926,10 @@ 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>CREATE STATISTICS</command> and
-         <command>ALTER TABLE VALIDATE</command> and other
-         <command>ALTER TABLE</command> variants (for full details see
-         <xref linkend="sql-altertable"/>).
+         <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
+         linkend="sql-altertable"/>).
         </para>
        </listitem>
       </varlistentry>
@@ -970,7 +970,7 @@ ERROR:  could not serialize access due to read/write dependencies among transact
         </para>
 
         <para>
-         Acquired by <command>CREATE TRIGGER</command> and many forms of
+         Acquired by <command>CREATE TRIGGER</command> and some forms of
          <command>ALTER TABLE</command> (see <xref linkend="sql-altertable"/>).
         </para>
        </listitem>
@@ -1020,7 +1020,7 @@ ERROR:  could not serialize access due to read/write dependencies among transact
          <command>CLUSTER</command>, <command>VACUUM FULL</command>,
          and <command>REFRESH MATERIALIZED VIEW</command> (without
          <option>CONCURRENTLY</option>)
-         commands. Many forms of <command>ALTER TABLE</command> also acquire
+         commands. Many forms of <command>ALTER INDEX</command> and <command>ALTER TABLE</command> also acquire
          a lock at this level. This is also the default lock mode for
          <command>LOCK TABLE</command> statements that do not specify
          a mode explicitly.
diff --git a/doc/src/sgml/ref/alter_index.sgml b/doc/src/sgml/ref/alter_index.sgml
index d0a62123583..6d34dbb74e5 100644
--- a/doc/src/sgml/ref/alter_index.sgml
+++ b/doc/src/sgml/ref/alter_index.sgml
@@ -39,7 +39,10 @@ ALTER INDEX ALL IN TABLESPACE <replaceable class="parameter">name</replaceable>
 
   <para>
    <command>ALTER INDEX</command> changes the definition of an existing index.
-   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>
 
@@ -53,6 +56,10 @@ ALTER INDEX ALL IN TABLESPACE <replaceable class="parameter">name</replaceable>
       or <literal>EXCLUDE</literal>), the constraint is renamed as well.
       There is no effect on the stored data.
      </para>
+     <para>
+      Renaming an index acquires a <literal>SHARE UPDATE EXCLUSIVE</literal>
+      lock.
+     </para>
     </listitem>
    </varlistentry>