Revert per-index collation version tracking feature.

Design problems were discovered in the handling of composite types and
record types that would cause some relevant versions not to be recorded.
Misgivings were also expressed about the use of the pg_depend catalog
for this purpose.  We're out of time for this release so we'll revert
and try again.

Commits reverted:

1bf946bd: Doc: Document known problem with Windows collation versions.
cf002008: Remove no-longer-relevant test case.
ef387bed: Fix bogus collation-version-recording logic.
0fb0a050: Hide internal error for pg_collation_actual_version(<bad OID>).
ff942057: Suppress "warning: variable 'collcollate' set but not used".
d50e3b1f: Fix assertion in collation version lookup.
f24b1569: Rethink extraction of collation dependencies.
257836a7: Track collation versions for indexes.
cd6f479e: Add pg_depend.refobjversion.
7d1297df: Remove pg_collation.collversion.

Discussion: https://postgr.es/m/CA%2BhUKGLhj5t1fcjqAu8iD9B3ixJtsTNqyCCD4V0aTO9kAKAjjA%40mail.gmail.com

5 files changed, 84 insertions, 39 deletions

diff --git a/doc/src/sgml/ref/alter_collation.sgml b/doc/src/sgml/ref/alter_collation.sgml
index 65429aabe28..af9ff2867b7 100644
--- a/doc/src/sgml/ref/alter_collation.sgml
+++ b/doc/src/sgml/ref/alter_collation.sgml
@@ -21,6 +21,8 @@ PostgreSQL documentation
 
  <refsynopsisdiv>
 <synopsis>
+ALTER COLLATION <replaceable>name</replaceable> REFRESH VERSION
+
 ALTER COLLATION <replaceable>name</replaceable> RENAME TO <replaceable>new_name</replaceable>
 ALTER COLLATION <replaceable>name</replaceable> OWNER TO { <replaceable>new_owner</replaceable> | CURRENT_ROLE | CURRENT_USER | SESSION_USER }
 ALTER COLLATION <replaceable>name</replaceable> SET SCHEMA <replaceable>new_schema</replaceable>
@@ -86,9 +88,70 @@ ALTER COLLATION <replaceable>name</replaceable> SET SCHEMA <replaceable>new_sche
     </listitem>
    </varlistentry>
 
+   <varlistentry>
+    <term><literal>REFRESH VERSION</literal></term>
+    <listitem>
+     <para>
+      Update the collation's version.
+      See <xref linkend="sql-altercollation-notes"/> below.
+     </para>
+    </listitem>
+   </varlistentry>
   </variablelist>
  </refsect1>
 
+ <refsect1 id="sql-altercollation-notes" xreflabel="Notes">
+  <title>Notes</title>
+
+  <para>
+   When using collations provided by the ICU library, the ICU-specific version
+   of the collator is recorded in the system catalog when the collation object
+   is created.  When the collation is used, the current version is
+   checked against the recorded version, and a warning is issued when there is
+   a mismatch, for example:
+<screen>
+WARNING:  collation "xx-x-icu" has version mismatch
+DETAIL:  The collation in the database was created using version 1.2.3.4, but the operating system provides version 2.3.4.5.
+HINT:  Rebuild all objects affected by this collation and run ALTER COLLATION pg_catalog."xx-x-icu" REFRESH VERSION, or build PostgreSQL with the right library version.
+</screen>
+   A change in collation definitions can lead to corrupt indexes and other
+   problems because the database system relies on stored objects having a
+   certain sort order.  Generally, this should be avoided, but it can happen
+   in legitimate circumstances, such as when
+   using <command>pg_upgrade</command> to upgrade to server binaries linked
+   with a newer version of ICU.  When this happens, all objects depending on
+   the collation should be rebuilt, for example,
+   using <command>REINDEX</command>.  When that is done, the collation version
+   can be refreshed using the command <literal>ALTER COLLATION ... REFRESH
+   VERSION</literal>.  This will update the system catalog to record the
+   current collator version and will make the warning go away.  Note that this
+   does not actually check whether all affected objects have been rebuilt
+   correctly.
+  </para>
+  <para>
+   When using collations provided by <literal>libc</literal> and
+   <productname>PostgreSQL</productname> was built with the GNU C library, the
+   C library's version is used as a collation version.  Since collation
+   definitions typically change only with GNU C library releases, this provides
+   some defense against corruption, but it is not completely reliable.
+  </para>
+  <para>
+   Currently, there is no version tracking for the database default collation.
+  </para>
+
+  <para>
+   The following query can be used to identify all collations in the current
+   database that need to be refreshed and the objects that depend on them:
+<programlisting><![CDATA[
+SELECT pg_describe_object(refclassid, refobjid, refobjsubid) AS "Collation",
+       pg_describe_object(classid, objid, objsubid) AS "Object"
+  FROM pg_depend d JOIN pg_collation c
+       ON refclassid = 'pg_collation'::regclass AND refobjid = c.oid
+  WHERE c.collversion <> pg_collation_actual_version(c.oid)
+  ORDER BY 1, 2;
+]]></programlisting></para>
+ </refsect1>
+
  <refsect1>
   <title>Examples</title>
 
diff --git a/doc/src/sgml/ref/alter_index.sgml b/doc/src/sgml/ref/alter_index.sgml
index 4b446384c26..e26efec064b 100644
--- a/doc/src/sgml/ref/alter_index.sgml
+++ b/doc/src/sgml/ref/alter_index.sgml
@@ -25,7 +25,6 @@ ALTER INDEX [ IF EXISTS ] <replaceable class="parameter">name</replaceable> RENA
 ALTER INDEX [ IF EXISTS ] <replaceable class="parameter">name</replaceable> SET TABLESPACE <replaceable class="parameter">tablespace_name</replaceable>
 ALTER INDEX <replaceable class="parameter">name</replaceable> ATTACH PARTITION <replaceable class="parameter">index_name</replaceable>
 ALTER INDEX <replaceable class="parameter">name</replaceable> [ NO ] DEPENDS ON EXTENSION <replaceable class="parameter">extension_name</replaceable>
-ALTER INDEX <replaceable class="parameter">name</replaceable> ALTER COLLATION <replaceable class="parameter">collation_name</replaceable> REFRESH VERSION
 ALTER INDEX [ IF EXISTS ] <replaceable class="parameter">name</replaceable> SET ( <replaceable class="parameter">storage_parameter</replaceable> [= <replaceable class="parameter">value</replaceable>] [, ... ] )
 ALTER INDEX [ IF EXISTS ] <replaceable class="parameter">name</replaceable> RESET ( <replaceable class="parameter">storage_parameter</replaceable> [, ... ] )
 ALTER INDEX [ IF EXISTS ] <replaceable class="parameter">name</replaceable> ALTER [ COLUMN ] <replaceable class="parameter">column_number</replaceable>
@@ -114,20 +113,6 @@ ALTER INDEX ALL IN TABLESPACE <replaceable class="parameter">name</replaceable>
    </varlistentry>
 
    <varlistentry>
-    <term><literal>ALTER COLLATION <replaceable class="parameter">collation_name</replaceable> REFRESH VERSION</literal></term>
-    <listitem>
-     <para>
-      Silences warnings about mismatched collation versions, by declaring
-      that the index is compatible with the current collation definition.
-      Be aware that incorrect use of this command can hide index corruption.
-      If you don't know whether a collation's definition has changed
-      incompatibly, <xref linkend="sql-reindex"/> is a safe alternative.
-      See <xref linkend="collation-versions"/> for more information.
-     </para>
-    </listitem>
-   </varlistentry>
-
-   <varlistentry>
     <term><literal>SET ( <replaceable class="parameter">storage_parameter</replaceable> [= <replaceable class="parameter">value</replaceable>] [, ... ] )</literal></term>
     <listitem>
      <para>
diff --git a/doc/src/sgml/ref/create_collation.sgml b/doc/src/sgml/ref/create_collation.sgml
index b97842071f9..58f5f0cd63a 100644
--- a/doc/src/sgml/ref/create_collation.sgml
+++ b/doc/src/sgml/ref/create_collation.sgml
@@ -27,6 +27,7 @@ CREATE COLLATION [ IF NOT EXISTS ] <replaceable>name</replaceable> (
     [ LC_CTYPE = <replaceable>lc_ctype</replaceable>, ]
     [ PROVIDER = <replaceable>provider</replaceable>, ]
     [ DETERMINISTIC = <replaceable>boolean</replaceable>, ]
+    [ VERSION = <replaceable>version</replaceable> ]
 )
 CREATE COLLATION [ IF NOT EXISTS ] <replaceable>name</replaceable> FROM <replaceable>existing_collation</replaceable>
 </synopsis>
@@ -149,6 +150,26 @@ CREATE COLLATION [ IF NOT EXISTS ] <replaceable>name</replaceable> FROM <replace
     </varlistentry>
 
     <varlistentry>
+     <term><replaceable>version</replaceable></term>
+
+     <listitem>
+      <para>
+       Specifies the version string to store with the collation.  Normally,
+       this should be omitted, which will cause the version to be computed
+       from the actual version of the collation as provided by the operating
+       system.  This option is intended to be used
+       by <command>pg_upgrade</command> for copying the version from an
+       existing installation.
+      </para>
+
+      <para>
+       See also <xref linkend="sql-altercollation"/> for how to handle
+       collation version mismatches.
+      </para>
+     </listitem>
+    </varlistentry>
+
+    <varlistentry>
      <term><replaceable>existing_collation</replaceable></term>
 
      <listitem>
diff --git a/doc/src/sgml/ref/pgupgrade.sgml b/doc/src/sgml/ref/pgupgrade.sgml
index 4737d97d202..a83c63cd98f 100644
--- a/doc/src/sgml/ref/pgupgrade.sgml
+++ b/doc/src/sgml/ref/pgupgrade.sgml
@@ -216,21 +216,6 @@ PostgreSQL documentation
      </varlistentry>
 
      <varlistentry>
-      <term><option>--index-collation-versions-unknown</option></term>
-      <listitem>
-       <para>
-        When upgrading indexes from releases before 14 that didn't track
-        collation versions, <application>pg_upgrade</application>
-        assumes by default that the upgraded indexes are compatible with the
-        currently installed versions of relevant collations (see
-        <xref linkend="collation-versions"/>).  Specify
-        <option>--index-collation-versions-unknown</option> to mark
-        them as needing to be rebuilt instead.
-       </para>
-      </listitem>
-     </varlistentry>
-
-     <varlistentry>
       <term><option>-?</option></term>
       <term><option>--help</option></term>
       <listitem><para>show help, then exit</para></listitem>
diff --git a/doc/src/sgml/ref/reindex.sgml b/doc/src/sgml/ref/reindex.sgml
index 53c362dcd3e..e6b25ee670f 100644
--- a/doc/src/sgml/ref/reindex.sgml
+++ b/doc/src/sgml/ref/reindex.sgml
@@ -42,15 +42,6 @@ REINDEX [ ( <replaceable class="parameter">option</replaceable> [, ...] ) ] { IN
    <itemizedlist>
     <listitem>
      <para>
-      The index depends on the sort order of a collation, and the definition
-      of the collation has changed.  This can cause index scans to fail to
-      find keys that are present.  See <xref linkend="collation-versions"/> for
-      more information.
-     </para>
-    </listitem>
-
-    <listitem>
-     <para>
       An index has become corrupted, and no longer contains valid
       data. Although in theory this should never happen, in
       practice indexes can become corrupted due to software bugs or