Change plan caching to honor, not resist, changes in search_path.

In the initial implementation of plan caching, we saved the active
search_path when a plan was first cached, then reinstalled that path
anytime we needed to reparse or replan.  The idea of that was to try to
reselect the same referenced objects, in somewhat the same way that views
continue to refer to the same objects in the face of schema or name
changes.  Of course, that analogy doesn't bear close inspection, since
holding the search_path fixed doesn't cope with object drops or renames.
Moreover sticking with the old path seems to create more surprises than
it avoids.  So instead of doing that, consider that the cached plan depends
on search_path, and force reparse/replan if the active search_path is
different than it was when we last saved the plan.

This gets us fairly close to having "transparency" of plan caching, in the
sense that the cached statement acts the same as if you'd just resubmitted
the original query text for another execution.  There are still some corner
cases where this fails though: a new object added in the search path
schema(s) might capture a reference in the query text, but we'd not realize
that and force a reparse.  We might try to fix that in the future, but for
the moment it looks too expensive and complicated.

doc/src/sgml/plpgsql.sgml

@@ -4258,7 +4258,9 @@ $$ LANGUAGE plpgsql;
     on specific parameter values, and caching that for re-use.  Typically
     this will happen only if the execution plan is not very sensitive to
     the values of the <application>PL/pgSQL</> variables referenced in it.
-    If it is, generating a plan each time is a net win.
+    If it is, generating a plan each time is a net win.  See <xref
+    linkend="sql-prepare"> for more information about the behavior of
+    prepared statements.
    </para>
 
    <para>

doc/src/sgml/ref/prepare.sgml

@@ -152,6 +152,28 @@ PREPARE <replaceable class="PARAMETER">name</replaceable> [ ( <replaceable class
    documentation.
   </para>
 
+  <para>
+   Although the main point of a prepared statement is to avoid repeated parse
+   analysis and planning of the statement, <productname>PostgreSQL</> will
+   force re-analysis and re-planning of the statement before using it
+   whenever database objects used in the statement have undergone
+   definitional (DDL) changes since the previous use of the prepared
+   statement.  Also, if the value of <xref linkend="guc-search-path"> changes
+   from one use to the next, the statement will be re-parsed using the new
+   <varname>search_path</>.  (This latter behavior is new as of
+   <productname>PostgreSQL</productname> 9.3.)  These rules make use of a
+   prepared statement semantically almost equivalent to re-submitting the
+   same query text over and over, but with a performance benefit if no object
+   definitions are changed, especially if the best plan remains the same
+   across uses.  An example of a case where the semantic equivalence is not
+   perfect is that if the statement refers to a table by an unqualified name,
+   and then a new table of the same name is created in a schema appearing
+   earlier in the <varname>search_path</>, no automatic re-parse will occur
+   since no object used in the statement changed.  However, if some other
+   change forces a re-parse, the new table will be referenced in subsequent
+   uses.
+  </para>
+
   <para>
    You can see all prepared statements available in the session by querying the
    <link linkend="view-pg-prepared-statements"><structname>pg_prepared_statements</structname></link>

doc/src/sgml/spi.sgml

@@ -975,6 +975,20 @@ SPIPlanPtr SPI_prepare(const char * <parameter>command</parameter>, int <paramet
    plans respectively.
   </para>
 
+  <para>
+   Although the main point of a prepared statement is to avoid repeated parse
+   analysis and planning of the statement, <productname>PostgreSQL</> will
+   force re-analysis and re-planning of the statement before using it
+   whenever database objects used in the statement have undergone
+   definitional (DDL) changes since the previous use of the prepared
+   statement.  Also, if the value of <xref linkend="guc-search-path"> changes
+   from one use to the next, the statement will be re-parsed using the new
+   <varname>search_path</>.  (This latter behavior is new as of
+   <productname>PostgreSQL</productname> 9.3.)  See <xref
+   linkend="sql-prepare"> for more information about the behavior of prepared
+   statements.
+  </para>
+
   <para>
    This function should only be called from a connected procedure.
   </para>