Rework the pg_statistic_ext catalog

Since extended statistic got introduced in PostgreSQL 10, there was a
single catalog pg_statistic_ext storing both the definitions and built
statistic.  That's however problematic when a user is supposed to have
access only to the definitions, but not to user data.

Consider for example pg_dump on a database with RLS enabled - if the
pg_statistic_ext catalog respects RLS (which it should, if it contains
user data), pg_dump would not see any records and the result would not
define any extended statistics.  That would be a surprising behavior.

Until now this was not a pressing issue, because the existing types of
extended statistic (functional dependencies and ndistinct coefficients)
do not include any user data directly.  This changed with introduction
of MCV lists, which do include most common combinations of values.

The easiest way to fix this is to split the pg_statistic_ext catalog
into two - one for definitions, one for the built statistic values.
The new catalog is called pg_statistic_ext_data, and we're maintaining
a 1:1 relationship with the old catalog - either there are matching
records in both catalogs, or neither of them.

Bumped CATVERSION due to changing system catalog definitions.

Author: Dean Rasheed, with improvements by me
Reviewed-by: Dean Rasheed, John Naylor
Discussion: https://postgr.es/m/CAEZATCUhT9rt7Ui%3DVdx4N%3D%3DVV5XOK5dsXfnGgVOz_JhAicB%3DZA%40mail.gmail.com

4 files changed, 76 insertions, 24 deletions

diff --git a/doc/src/sgml/catalogs.sgml b/doc/src/sgml/catalogs.sgml
index 36193d14910..ef4345524ad 100644
--- a/doc/src/sgml/catalogs.sgml
+++ b/doc/src/sgml/catalogs.sgml
@@ -297,7 +297,12 @@
 
      <row>
       <entry><link linkend="catalog-pg-statistic-ext"><structname>pg_statistic_ext</structname></link></entry>
-      <entry>extended planner statistics</entry>
+      <entry>extended planner statistics (definition)</entry>
+     </row>
+
+     <row>
+      <entry><link linkend="catalog-pg-statistic-ext-data"><structname>pg_statistic_ext_data</structname></link></entry>
+      <entry>extended planner statistics (built statistics)</entry>
      </row>
 
      <row>
@@ -6506,7 +6511,7 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</replaceable>:<replaceable>&l
 
   <para>
    The catalog <structname>pg_statistic_ext</structname>
-   holds extended planner statistics.
+   holds definitions of extended planner statistics.
    Each row in this catalog corresponds to a <firstterm>statistics object</firstterm>
    created with <xref linkend="sql-createstatistics"/>.
   </para>
@@ -6581,8 +6586,57 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</replaceable>:<replaceable>&l
       </entry>
      </row>
 
+    </tbody>
+   </tgroup>
+  </table>
+
+  <para>
+   The <structfield>stxkind</structfield> field is filled at creation of the
+   statistics object, indicating which statistic type(s) are desired. The
+   statistics (once computed by <command>ANALYZE</command>) are stored in
+   <link linkend="catalog-pg-statistic-ext-data"><structname>pg_statistic_ext_data</structname></link>
+   catalog.
+  </para>
+ </sect1>
+
+ <sect1 id="catalog-pg-statistic-ext-data">
+  <title><structname>pg_statistic_ext_data</structname></title>
+
+  <indexterm zone="catalog-pg-statistic-ext">
+   <primary>pg_statistic_ext_data</primary>
+  </indexterm>
+
+  <para>
+   The catalog <structname>pg_statistic_ext_data</structname>
+   holds data for extended planner statistics defined in <structname>pg_statistic_ext</structname>.
+   Each row in this catalog corresponds to a <firstterm>statistics object</firstterm>
+   created with <xref linkend="sql-createstatistics"/>.
+  </para>
+
+  <table>
+   <title><structname>pg_statistic_ext_data</structname> Columns</title>
+
+   <tgroup cols="4">
+    <thead>
+     <row>
+      <entry>Name</entry>
+      <entry>Type</entry>
+      <entry>References</entry>
+      <entry>Description</entry>
+     </row>
+    </thead>
+
+    <tbody>
+
+     <row>
+      <entry><structfield>stxoid</structfield></entry>
+      <entry><type>oid</type></entry>
+      <entry><literal><link linkend="catalog-pg-statistic-ext"><structname>pg_statistic_ext</structname></link>.oid</literal></entry>
+      <entry>Extended statistic containing the definition for this data.</entry>
+     </row>
+
      <row>
-      <entry><structfield>stxndistinct</structfield></entry>
+      <entry><structfield>stxdndistinct</structfield></entry>
       <entry><type>pg_ndistinct</type></entry>
       <entry></entry>
       <entry>
@@ -6591,7 +6645,7 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</replaceable>:<replaceable>&l
      </row>
 
      <row>
-      <entry><structfield>stxdependencies</structfield></entry>
+      <entry><structfield>stxddependencies</structfield></entry>
       <entry><type>pg_dependencies</type></entry>
       <entry></entry>
       <entry>
@@ -6601,7 +6655,7 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</replaceable>:<replaceable>&l
      </row>
 
      <row>
-      <entry><structfield>stxmcv</structfield></entry>
+      <entry><structfield>stxdmcv</structfield></entry>
       <entry><type>pg_mcv_list</type></entry>
       <entry></entry>
       <entry>
@@ -6614,12 +6668,6 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</replaceable>:<replaceable>&l
    </tgroup>
   </table>
 
-  <para>
-   The <structfield>stxkind</structfield> field is filled at creation of the
-   statistics object, indicating which statistic type(s) are desired.
-   The fields after it are initially NULL and are filled only when the
-   corresponding statistic has been computed by <command>ANALYZE</command>.
-  </para>
  </sect1>
 
  <sect1 id="catalog-pg-subscription">
diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index a072b976161..e9181338742 100644
--- a/doc/src/sgml/func.sgml
+++ b/doc/src/sgml/func.sgml
@@ -22427,12 +22427,12 @@ CREATE EVENT TRIGGER test_table_rewrite_oid
     The <function>pg_mcv_list_items</function> function can be used like this:
 
 <programlisting>
-SELECT m.* FROM pg_statistic_ext,
-                pg_mcv_list_items(stxmcv) m WHERE stxname = 'stts';
+SELECT m.* FROM pg_statistic_ext join pg_statistic_ext_data on (oid = stxoid),
+                pg_mcv_list_items(stxdmcv) m WHERE stxname = 'stts';
 </programlisting>
 
      Values of the <type>pg_mcv_list</type> can be obtained only from the
-     <literal>pg_statistic_ext.stxmcv</literal> column.
+     <literal>pg_statistic_ext_data.stxdmcv</literal> column.
    </para>
   </sect2>
 
diff --git a/doc/src/sgml/perform.sgml b/doc/src/sgml/perform.sgml
index a84be851593..8e165832b35 100644
--- a/doc/src/sgml/perform.sgml
+++ b/doc/src/sgml/perform.sgml
@@ -1076,6 +1076,10 @@ WHERE tablename = 'road';
     <primary>pg_statistic_ext</primary>
    </indexterm>
 
+   <indexterm>
+    <primary>pg_statistic_ext_data</primary>
+   </indexterm>
+
    <para>
     It is common to see slow queries running bad execution plans because
     multiple columns used in the query clauses are correlated.
@@ -1104,7 +1108,7 @@ WHERE tablename = 'road';
     interest in the statistics.  Actual data collection is performed
     by <command>ANALYZE</command> (either a manual command, or background
     auto-analyze).  The collected values can be examined in the
-    <link linkend="catalog-pg-statistic-ext"><structname>pg_statistic_ext</structname></link>
+    <link linkend="catalog-pg-statistic-ext-data"><structname>pg_statistic_ext_data</structname></link>
     catalog.
    </para>
 
@@ -1172,10 +1176,10 @@ CREATE STATISTICS stts (dependencies) ON zip, city FROM zipcodes;
 
 ANALYZE zipcodes;
 
-SELECT stxname, stxkeys, stxdependencies
-  FROM pg_statistic_ext
+SELECT stxname, stxkeys, stxddependencies
+  FROM pg_statistic_ext join pg_statistic_ext_data on (oid = stxoid)
   WHERE stxname = 'stts';
- stxname | stxkeys |             stxdependencies               
+ stxname | stxkeys |             stxddependencies             
 ---------+---------+------------------------------------------
  stts    | 1 5     | {"1 => 5": 1.000000, "5 => 1": 0.423130}
 (1 row)
@@ -1262,8 +1266,8 @@ CREATE STATISTICS stts2 (ndistinct) ON zip, state, city FROM zipcodes;
 
 ANALYZE zipcodes;
 
-SELECT stxkeys AS k, stxndistinct AS nd
-  FROM pg_statistic_ext
+SELECT stxkeys AS k, stxdndistinct AS nd
+  FROM pg_statistic_ext join pg_statistic_ext_data on (oid = stxoid)
   WHERE stxname = 'stts2';
 -[ RECORD 1 ]--------------------------------------------------------
 k  | 1 2 5
@@ -1317,8 +1321,8 @@ CREATE STATISTICS stts3 (mcv) ON state, city FROM zipcodes;
 
 ANALYZE zipcodes;
 
-SELECT m.* FROM pg_statistic_ext,
-                pg_mcv_list_items(stxmcv) m WHERE stxname = 'stts3';
+SELECT m.* FROM pg_statistic_ext join pg_statistic_ext_data on (oid = stxoid),
+                pg_mcv_list_items(stxdmcv) m WHERE stxname = 'stts3';
 
  index |         values         | nulls | frequency | base_frequency 
 -------+------------------------+-------+-----------+----------------
diff --git a/doc/src/sgml/planstats.sgml b/doc/src/sgml/planstats.sgml
index 4b1d3f4952e..a25ce152ac2 100644
--- a/doc/src/sgml/planstats.sgml
+++ b/doc/src/sgml/planstats.sgml
@@ -635,8 +635,8 @@ EXPLAIN (ANALYZE, TIMING OFF) SELECT * FROM t WHERE a = 1 AND b = 1;
     <function>pg_mcv_list_items</function> set-returning function.
 
 <programlisting>
-SELECT m.* FROM pg_statistic_ext,
-                pg_mcv_list_items(stxmcv) m WHERE stxname = 'stts2';
+SELECT m.* FROM pg_statistic_ext join pg_statistic_ext_data on (oid = stxoid),
+                pg_mcv_list_items(stxdmcv) m WHERE stxname = 'stts2';
  index |  values  | nulls | frequency | base_frequency 
 -------+----------+-------+-----------+----------------
      0 | {0, 0}   | {f,f} |      0.01 |         0.0001