doc: Add best practises section to partitioning docs

A few questionable partitioning designs have been cropping up lately
around the mailing lists.  Generally, these cases have been partitioning
using too many partitions which have caused performance or OOM problems for
the users.

Since we have very little else to guide users into good design, here we
add a new section to the partitioning documentation with some best
practise guidelines for good design.

Reviewed-by: Justin Pryzby, Amit Langote, Alvaro Herrera
Discussion: https://postgr.es/m/CAKJS1f-2rx+E9mG3xrCVHupefMjAp1+tpczQa9SEOZWyU7fjEA@mail.gmail.com
Backpatch-through: 10

Author: david-rowley

doc/src/sgml/ddl.sgml

@@ -3450,8 +3450,9 @@ VALUES ('Albany', NULL, NULL, 'NY');
     </listitem>
    </itemizedlist>
 
-   These deficiencies will probably be fixed in some future release,
-   but in the meantime considerable care is needed in deciding whether
+   Some functionality not implemented for inheritance hierarchies is
+   implemented for declarative partitioning.
+   Considerable care is needed in deciding whether partitioning with legacy
    inheritance is useful for your application.
   </para>
 
@@ -4674,6 +4675,87 @@ EXPLAIN SELECT count(*) FROM measurement WHERE logdate &gt;= DATE '2008-01-01';
    </itemizedlist>
    </para>
   </sect2>
+  
+  <sect2 id="ddl-partitioning-declarative-best-practices">
+   <title>Declarative Partitioning Best Practices</title>
+
+   <para>
+    The choice of how to partition a table should be made carefully as the
+    performance of query planning and execution can be negatively affected by
+    poor design.
+   </para>
+
+   <para>
+    One of the most critical design decisions will be the column or columns
+    by which you partition your data.  Often the best choice will be to
+    partition by the column or set of columns which most commonly appear in
+    <literal>WHERE</literal> clauses of queries being executed on the
+    partitioned table.  <literal>WHERE</literal> clause items that match and
+    are compatible with the partition key can be used to prune unneeded
+    partitions.  However, you may be forced into making other decisions by
+    requirements for the <literal>PRIMARY KEY</literal> or a
+    <literal>UNIQUE</literal> constraint.  Removal of unwanted data is also a
+    factor to consider when planning your partitioning strategy.  An entire
+    partition can be detached fairly quickly, so it may be beneficial to
+    design the partition strategy in such a way that all data to be removed
+    at once is located in a single partition.
+   </para>
+
+   <para>
+    Choosing the target number of partitions that the table should be divided
+    into is also a critical decision to make.  Not having enough partitions
+    may mean that indexes remain too large and that data locality remains poor
+    which could result in low cache hit ratios.  However, dividing the table
+    into too many partitions can also cause issues.  Too many partitions can
+    mean longer query planning times and higher memory consumption during both
+    query planning and execution.  When choosing how to partition your table,
+    it's also important to consider what changes may occur in the future.  For
+    example, if you choose to have one partition per customer and you
+    currently have a small number of large customers, consider the
+    implications if in several years you instead find yourself with a large
+    number of small customers.  In this case, it may be better to choose to
+    partition by <literal>HASH</literal> and choose a reasonable number of
+    partitions rather than trying to partition by <literal>LIST</literal> and
+    hoping that the number of customers does not increase beyond what it is
+    practical to partition the data by.
+   </para>
+
+   <para>
+    Sub-partitioning can be useful to further divide partitions that are
+    expected to become larger than other partitions, although excessive
+    sub-partitioning can easily lead to large numbers of partitions and can
+    cause the same problems mentioned in the preceding paragraph.
+   </para>
+
+   <para>
+    It is also important to consider the overhead of partitioning during
+    query planning and execution.  The query planner is generally able to
+    handle partition hierarchies up a few thousand partitions fairly well,
+    provided that typical queries allow the query planner to prune all but a
+    small number of partitions.  Planning times become longer and memory
+    consumption becomes higher when more partitions remain after the planner
+    performs partition pruning.  This is particularly true for the
+    <command>UPDATE</command> and <command>DELETE</command> commands. Another
+    reason to be concerned about having a large number of partitions is that
+    the server's memory consumption may grow significantly over a period of
+    time, especially if many sessions touch large numbers of partitions.
+    That's because each partition requires its metadata to be loaded into the
+    local memory of each session that touches it.
+   </para>
+
+   <para>
+    With data warehouse type workloads, it can make sense to use a larger
+    number of partitions than with an <acronym>OLTP</acronym> type workload.
+    Generally, in data warehouses, query planning time is less of a concern as
+    the majority of processing time is spent during query execution.  With
+    either of these two types of workload, it is important to make the right
+    decisions early, as re-partitioning large quantities of data can be
+    painfully slow.  Simulations of the intended workload are often beneficial
+    for optimizing the partitioning strategy.  Never assume that more
+    partitions are better than fewer partitions and vice-versa.
+   </para>
+  </sect2>
+
  </sect1>
 
  <sect1 id="ddl-foreign-data">