Implement multivariate n-distinct coefficients

Add support for explicitly declared statistic objects (CREATE
STATISTICS), allowing collection of statistics on more complex
combinations that individual table columns.  Companion commands DROP
STATISTICS and ALTER STATISTICS ... OWNER TO / SET SCHEMA / RENAME are
added too.  All this DDL has been designed so that more statistic types
can be added later on, such as multivariate most-common-values and
multivariate histograms between columns of a single table, leaving room
for permitting columns on multiple tables, too, as well as expressions.

This commit only adds support for collection of n-distinct coefficient
on user-specified sets of columns in a single table.  This is useful to
estimate number of distinct groups in GROUP BY and DISTINCT clauses;
estimation errors there can cause over-allocation of memory in hashed
aggregates, for instance, so it's a worthwhile problem to solve.  A new
special pseudo-type pg_ndistinct is used.

(num-distinct estimation was deemed sufficiently useful by itself that
this is worthwhile even if no further statistic types are added
immediately; so much so that another version of essentially the same
functionality was submitted by Kyotaro Horiguchi:
https://postgr.es/m/20150828.173334.114731693.horiguchi.kyotaro@lab.ntt.co.jp
though this commit does not use that code.)

Author: Tomas Vondra.  Some code rework by Álvaro.
Reviewed-by: Dean Rasheed, David Rowley, Kyotaro Horiguchi, Jeff Janes,
    Ideriha Takeshi
Discussion: https://postgr.es/m/543AFA15.4080608@fuzzy.cz
    https://postgr.es/m/20170320190220.ixlaueanxegqd5gr@alvherre.pgsql

1 files changed, 98 insertions, 0 deletions

diff --git a/doc/src/sgml/ref/drop_statistics.sgml b/doc/src/sgml/ref/drop_statistics.sgml
new file mode 100644
index 00000000000..98c338182b4
--- /dev/null
+++ b/doc/src/sgml/ref/drop_statistics.sgml
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+<!--
+doc/src/sgml/ref/drop_statistics.sgml
+PostgreSQL documentation
+-->
+
+<refentry id="SQL-DROPSTATISTICS">
+ <indexterm zone="sql-dropstatistics">
+  <primary>DROP STATISTICS</primary>
+ </indexterm>
+
+ <refmeta>
+  <refentrytitle>DROP STATISTICS</refentrytitle>
+  <manvolnum>7</manvolnum>
+  <refmiscinfo>SQL - Language Statements</refmiscinfo>
+ </refmeta>
+
+ <refnamediv>
+  <refname>DROP STATISTICS</refname>
+  <refpurpose>remove extended statistics</refpurpose>
+ </refnamediv>
+
+ <refsynopsisdiv>
+<synopsis>
+DROP STATISTICS [ IF EXISTS ] <replaceable class="PARAMETER">name</replaceable> [, ...]
+</synopsis>
+ </refsynopsisdiv>
+
+ <refsect1>
+  <title>Description</title>
+
+  <para>
+   <command>DROP STATISTICS</command> removes statistics from the database.
+   Only the statistics owner, the schema owner, and superuser can drop a
+   statistics.
+  </para>
+
+ </refsect1>
+
+ <refsect1>
+  <title>Parameters</title>
+
+  <variablelist>
+   <varlistentry>
+    <term><literal>IF EXISTS</literal></term>
+    <listitem>
+     <para>
+      Do not throw an error if the statistics do not exist. A notice is
+      issued in this case.
+     </para>
+    </listitem>
+   </varlistentry>
+
+   <varlistentry>
+    <term><replaceable class="PARAMETER">name</replaceable></term>
+    <listitem>
+     <para>
+      The name (optionally schema-qualified) of the statistics to drop.
+     </para>
+    </listitem>
+   </varlistentry>
+
+  </variablelist>
+ </refsect1>
+
+ <refsect1>
+  <title>Examples</title>
+
+  <para>
+   To destroy two statistics objects on different schemas, without failing
+   if they don't exist:
+
+<programlisting>
+DROP STATISTICS IF EXISTS
+    accounting.users_uid_creation,
+    public.grants_user_role;
+</programlisting>
+  </para>
+
+ </refsect1>
+
+ <refsect1>
+  <title>Compatibility</title>
+
+  <para>
+   There's no <command>DROP STATISTICS</command> command in the SQL standard.
+  </para>
+ </refsect1>
+
+ <refsect1>
+  <title>See Also</title>
+
+  <simplelist type="inline">
+   <member><xref linkend="sql-alterstatistics"></member>
+   <member><xref linkend="sql-createstatistics"></member>
+  </simplelist>
+ </refsect1>
+
+</refentry>