doc: Add section for temporal tables

This section introduces temporal tables, with a focus on Application
Time (which we support) and only a brief mention of System Time (which
we don't).  It covers temporal primary keys, unique constraints, and
temporal foreign keys.  We will document temporal update/delete and
periods as we add those features.

This commit also adds glossary entries for temporal table, application
time, and system time.

Author: Paul A. Jungwirth <pj@illuminatedcomputing.com>
Discussion: https://www.postgresql.org/message-id/flat/ec498c3d-5f2b-48ec-b989-5561c8aa2024@illuminatedcomputing.com

7 files changed, 446 insertions, 1 deletions

diff --git a/doc/src/sgml/ddl.sgml b/doc/src/sgml/ddl.sgml
index 65bc070d2e5..e199b479867 100644
--- a/doc/src/sgml/ddl.sgml
+++ b/doc/src/sgml/ddl.sgml
@@ -1585,6 +1585,298 @@ CREATE TABLE circles (
    </para>
  </sect1>
 
+ <sect1 id="ddl-temporal-tables">
+  <title>Temporal Tables</title>
+
+  <indexterm zone="ddl-temporal-tables">
+   <primary>temporal</primary>
+  </indexterm>
+
+  <para>
+   <firstterm>Temporal tables</firstterm> allow users to track different
+   dimensions of history.  <firstterm>Application time</firstterm> tracks the
+   history of a thing out in the world, and <firstterm>system time</firstterm>
+   tracks the history of the database itself.  (A database that does both is
+   also called <firstterm>bitemporal</firstterm>.)  This section describes how
+   to express and manage such histories in temporal tables.
+  </para>
+
+  <sect2 id="ddl-application-time">
+   <title>Application Time</title>
+
+   <indexterm zone="ddl-application-time">
+    <primary>application time</primary>
+   </indexterm>
+
+   <para>
+    <firstterm>Application time</firstterm> refers to a history of the entity
+    described by a table.  In a typical non-temporal table, there is single
+    row for each entity.  In a temporal table, an entity may have multiple
+    rows, as long as those rows describe non-overlapping periods from its
+    history.  Application time requires each row to have a start and end time,
+    expressing when the row is applicable.
+   </para>
+
+   <para>
+    The following SQL creates a temporal table that can store application time:
+<programlisting>
+CREATE TABLE products (
+    product_no integer,
+    price numeric,
+    <emphasis>valid_at daterange</emphasis>
+);
+</programlisting>
+   </para>
+
+   <para>
+    Records in a temporal table can be imagined on a timeline, as in <xref
+    linkend="temporal-entities-figure"/>.  Here we show three records
+    describing two products.  Each record is a tuple with three attributes:
+    the product number, the price, and the application time.  So product 5 was
+    first offered for a price of 5.00 starting January 1, 2020, but then
+    became 8.00 starting January 1, 2022.  Its second record has no specified
+    end time, indicating that it is true indefinitely, or for all future time.
+    The last record shows that product 6 was introduced January 1, 2021 for
+    9.00, then canceled January 1, 2024.
+   </para>
+
+   <figure id="temporal-entities-figure">
+    <title>Application Time Example</title>
+    <mediaobject>
+     <imageobject>
+      <imagedata fileref="images/temporal-entities.svg" format="SVG" width="100%"/>
+     </imageobject>
+    </mediaobject>
+   </figure>
+
+   <para>
+    In a table, these records would be:
+<programlisting>
+ product_no | price |        valid_at
+------------+-------+-------------------------
+          5 |  5.00 | [2020-01-01,2022-01-01)
+          5 |  8.00 | [2022-01-01,)
+          6 |  9.00 | [2021-01-01,2024-01-01)
+</programlisting>
+   </para>
+
+   <para>
+    We show the application time using range-type notation, because it is
+    stored as a single column (either a range or multirange).  Ranges include
+    their start point but exclude their end point.  That way two adjacent
+    ranges cover all points without overlapping.  See <xref
+    linkend="rangetypes"/> for more information about range types.
+   </para>
+
+   <para>
+    In principle, a table with application-time ranges/multiranges is
+    equivalent to a table that stores application-time
+    <quote>instants</quote>: one for each second, millisecond, nanosecond, or
+    whatever finest granularity is available.  But such a table would contain
+    far too many rows, so ranges/multiranges offer an optimization to
+    represent the same information in a compact form.  In addition, ranges and
+    multiranges offer a more convenient interface for typical temporal
+    operations, where records change infrequently enough that separate
+    <quote>versions</quote> persist for extended periods of time.
+   </para>
+
+   <sect3 id="ddl-application-time-primary-keys">
+    <title>Temporal Primary Keys and Unique Constraints</title>
+
+    <para>
+     A table with application time has a different concept of entity
+     uniqueness than a non-temporal table.  Temporal entity uniqueness can be
+     enforced with a temporal primary key.  A regular primary key has at least
+     one column, all columns are <literal>NOT NULL</literal>, and the combined
+     value of all columns is unique.  A temporal primary key also has at least
+     one such column, but in addition it has a final column that is of a range
+     type or multirange type that shows when the row is applicable.  The
+     regular parts of the key must be unique for any moment in time, but
+     non-unique rows are allowed if their application time does not overlap.
+    </para>
+
+    <para>
+     The syntax to create a temporal primary key is as follows:
+
+<programlisting>
+CREATE TABLE products (
+    product_no integer,
+    price numeric,
+    valid_at daterange,
+    <emphasis>PRIMARY KEY (product_no, valid_at WITHOUT OVERLAPS)</emphasis>
+);
+</programlisting>
+
+     In this example, <literal>product_no</literal> is the non-temporal part
+     of the key, and <literal>valid_at</literal> is a range column containing
+     the application time.
+    </para>
+
+    <para>
+     The <literal>WITHOUT OVERLAPS</literal> column is implicitly <literal>NOT
+     NULL</literal> (like the other parts of the key).  In addition it may not
+     contain empty values, that is, a range of <literal>'empty'</literal> or a
+     multirange of <literal>{}</literal>.  An empty application time would
+     have no meaning.
+    </para>
+
+    <para>
+     It is also possible to create a temporal unique constraint that is
+     not a primary key. The syntax is similar:
+
+<programlisting>
+CREATE TABLE products (
+    product_no integer,
+    price numeric,
+    valid_at daterange,
+    <emphasis>UNIQUE (product_no, valid_at WITHOUT OVERLAPS)</emphasis>
+);
+</programlisting>
+
+     Temporal unique constraints also forbid empty ranges/multiranges for
+     their application time, but that column is permitted to be null (like the
+     other columns of the unique constraint).
+    </para>
+
+    <para>
+     Temporal primary keys and unique constraints are backed by GiST indexes
+     (see <xref linkend="gist"/>) rather than B-Tree indexes.  In practice,
+     creating a temporal primary key or constraint requires installing the
+     <xref linkend="btree-gist"/> extension, so that the database has GiST
+     operator classes for the non-temporal parts of the key.
+    </para>
+
+    <para>
+     Temporal primary keys and unique constraints have the same behavior as
+     exclusion constraints (see <xref linkend="ddl-constraints-exclusion"/>),
+     where each regular key part is compared with equality, and the
+     application time is compared with overlaps, for example <literal>EXCLUDE
+     USING gist (id WITH =, valid_at WITH &amp;&amp;)</literal>. The only
+     difference is that they also forbid an empty application time.
+    </para>
+   </sect3>
+
+   <sect3 id="ddl-application-time-foreign-keys">
+    <title>Temporal Foreign Keys</title>
+
+    <para>
+     A temporal foreign key is a reference from one application-time table to
+     another application-time table.  Just as a non-temporal reference
+     requires a referenced key to exist, so a temporal reference requires a
+     referenced key to exist, but during whatever history the reference exists
+     (at least).  So if the <literal>products</literal> table is referenced by
+     a <literal>variants</literal> table, and a variant of product 5 has an
+     application-time of <literal>[2020-01-01,2026-01-01)</literal>, then
+     product 5 must exist throughout that period.
+    </para>
+
+    <para>
+     We can create the <literal>variants</literal> table with the following
+     schema (without a foreign key yet):
+
+<programlisting>
+CREATE TABLE variants (
+  id         integer,
+  product_no integer,
+  name       text,
+  valid_at   daterange,
+  PRIMARY KEY (id, valid_at WITHOUT OVERLAPS)
+);
+</programlisting>
+
+     We have included a temporal primary key as a best practice, but it is not
+     strictly required by foreign keys.
+    </para>
+
+    <para>
+     <xref linkend="temporal-references-figure"/> plots product 5 (in green)
+     and two variants referencing it (in yellow) on the same timeline.
+     Variant 8 (Medium) was introduced first, then variant 9 (XXL).  Both
+     satisfy the foreign key constraint, because the referenced product exists
+     throughout their entire history.
+    </para>
+
+    <figure id="temporal-references-figure">
+     <title>Temporal Foreign Key Example</title>
+     <mediaobject>
+      <imageobject>
+       <imagedata fileref="images/temporal-references.svg" format="SVG" width="100%"/>
+      </imageobject>
+     </mediaobject>
+    </figure>
+
+    <para>
+
+     In a table, these records would be:
+<programlisting>
+ id | product_no |  name  |        valid_at
+----+------------+--------+-------------------------
+  8 |          5 | Medium | [2021-01-01,2023-06-01)
+  9 |          5 | XXL    | [2022-03-01,2024-06-01)
+</programlisting>
+    </para>
+
+    <para>
+     Note that a temporal reference need not be fulfilled by a single row in
+     the referenced table.  Product 5 had a price change in the middle of
+     variant 8's history, but the reference is still valid.  The combination
+     of all matching rows is used to test whether the referenced history
+     contains the referencing row.
+    </para>
+
+    <para>
+     The syntax to add a temporal foreign key to our table is:
+
+<programlisting>
+CREATE TABLE variants (
+  id         integer,
+  product_no integer,
+  name       text,
+  valid_at   daterange,
+  PRIMARY KEY (id, valid_at WITHOUT OVERLAPS),
+  <emphasis>FOREIGN KEY (product_no, PERIOD valid_at) REFERENCES products (product_no, PERIOD valid_at)</emphasis>
+);
+</programlisting>
+
+     Note that the keyword <literal>PERIOD</literal> must be used for the
+     application-time column in both the referencing and referenced table.
+    </para>
+
+    <para>
+     A temporal primary key or unique constraint matching the referenced columns
+     must exist on the referenced table.
+    </para>
+
+    <para>
+     <productname>PostgreSQL</productname> supports temporal foreign keys with
+     action <literal>NO ACTION</literal>, but not <literal>RESTRICT</literal>,
+     <literal>CASCADE</literal>, <literal>SET NULL</literal>, or <literal>SET
+     DEFAULT</literal>.
+    </para>
+   </sect3>
+  </sect2>
+
+  <sect2 id="ddl-system-time">
+   <title>System Time</title>
+
+   <indexterm zone="ddl-system-time">
+    <primary>system time</primary>
+   </indexterm>
+
+   <para>
+    <firstterm>System time</firstterm> refers to the history of the database
+    table, not the entity it describes.  It captures when each row was
+    inserted/updated/deleted.
+   </para>
+
+   <para>
+    <productname>PostgreSQL</productname> does not currently support system
+    time, but it could be emulated using triggers, and there are external
+    extensions that provide such functionality.
+   </para>
+  </sect2>
+ </sect1>
+
  <sect1 id="ddl-alter">
   <title>Modifying Tables</title>
 
diff --git a/doc/src/sgml/glossary.sgml b/doc/src/sgml/glossary.sgml
index 8651f0cdb91..a76cf5c383f 100644
--- a/doc/src/sgml/glossary.sgml
+++ b/doc/src/sgml/glossary.sgml
@@ -81,6 +81,21 @@
    </glossdef>
   </glossentry>
 
+  <glossentry id="glossary-application-time">
+   <glossterm>Application time</glossterm>
+   <glossdef>
+    <para>
+     In a <glossterm linkend="glossary-temporal-table">temporal table</glossterm>,
+     the dimension of time that represents when the entity described by the table
+     changed (as opposed to the table itself).
+    </para>
+    <para>
+     For more information, see
+     <xref linkend="ddl-temporal-tables"/>.
+    </para>
+   </glossdef>
+  </glossentry>
+
   <glossentry id="glossary-aio">
    <glossterm>Asynchronous <acronym>I/O</acronym></glossterm>
    <acronym>AIO</acronym>
@@ -1847,6 +1862,22 @@
    </glossdef>
   </glossentry>
 
+  <glossentry id="glossary-system-time">
+   <glossterm>System time</glossterm>
+   <glossdef>
+    <para>
+     In a <glossterm linkend="glossary-temporal-table">temporal table</glossterm>,
+     the dimension of time that represents when the table itself was changed
+     (as opposed to the entity the table describes).
+     Often used for auditing, compliance, and debugging.
+    </para>
+    <para>
+     For more information, see
+     <xref linkend="ddl-temporal-tables"/>.
+    </para>
+   </glossdef>
+  </glossentry>
+
   <glossentry id="glossary-table">
    <glossterm>Table</glossterm>
    <glossdef>
@@ -1885,6 +1916,22 @@
    </glossdef>
   </glossentry>
 
+  <glossentry id="glossary-temporal-table">
+   <glossterm>Temporal table</glossterm>
+   <glossdef>
+    <para>
+     <glossterm linkend="glossary-table">Tables</glossterm>
+     that track <glossterm linkend="glossary-application-time">application time</glossterm>
+     or <glossterm linkend="glossary-system-time">system time</glossterm> (or both).
+     Not to be confused with <glossterm linkend="glossary-temporary-table">temporary tables</glossterm>.
+    </para>
+    <para>
+     For more information, see
+     <xref linkend="ddl-temporal-tables"/>.
+    </para>
+   </glossdef>
+  </glossentry>
+
   <glossentry id="glossary-temporary-table">
    <glossterm>Temporary table</glossterm>
    <glossdef>
diff --git a/doc/src/sgml/images/Makefile b/doc/src/sgml/images/Makefile
index 645519095d0..fd55b9ad23f 100644
--- a/doc/src/sgml/images/Makefile
+++ b/doc/src/sgml/images/Makefile
@@ -5,7 +5,9 @@
 ALL_IMAGES = \
 	genetic-algorithm.svg \
 	gin.svg \
-	pagelayout.svg
+	pagelayout.svg \
+	temporal-entities.svg \
+	temporal-references.svg
 
 DITAA = ditaa
 DOT = dot
diff --git a/doc/src/sgml/images/temporal-entities.svg b/doc/src/sgml/images/temporal-entities.svg
new file mode 100644
index 00000000000..23958c3203c
--- /dev/null
+++ b/doc/src/sgml/images/temporal-entities.svg
@@ -0,0 +1,34 @@
+<?xml version="1.0"?>
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 1020 252" width="1020" height="252" shape-rendering="geometricPrecision" version="1.0">
+  <defs>
+    <filter id="f2" x="0" y="0" width="200%" height="200%">
+      <feOffset result="offOut" in="SourceGraphic" dx="5" dy="5"/>
+      <feGaussianBlur result="blurOut" in="offOut" stdDeviation="3"/>
+      <feBlend in="SourceGraphic" in2="blurOut" mode="normal"/>
+    </filter>
+  </defs>
+  <g stroke-width="1" stroke-linecap="square" stroke-linejoin="round">
+    <rect x="0" y="0" width="1020" height="252" style="fill: #ffffff"/>
+    <path stroke="#000000" stroke-width="1.000000" stroke-linecap="round" stroke-linejoin="round" fill="#99dd99" d="M215.0 105.0 L215.0 175.0 L785.0 175.0 L785.0 105.0 z"/>
+    <path stroke="#000000" stroke-width="1.000000" stroke-linecap="round" stroke-linejoin="round" fill="#99dd99" d="M965.0 35.0 L965.0 105.0 L405.0 105.0 L405.0 35.0 z"/>
+    <path stroke="#000000" stroke-width="1.000000" stroke-linecap="round" stroke-linejoin="round" fill="#99dd99" d="M25.0 35.0 L25.0 105.0 L405.0 105.0 L405.0 35.0 z"/>
+    <path stroke="#000000" stroke-width="1.000000" stroke-linecap="round" stroke-linejoin="round" fill="none" d="M25.0 196.0 L25.0 209.0 "/>
+    <path stroke="#000000" stroke-width="1.000000" stroke-linecap="round" stroke-linejoin="round" fill="none" d="M405.0 196.0 L405.0 209.0 "/>
+    <path stroke="#000000" stroke-width="1.000000" stroke-linecap="round" stroke-linejoin="round" fill="none" d="M215.0 196.0 L215.0 209.0 "/>
+    <path stroke="#000000" stroke-width="1.000000" stroke-linecap="round" stroke-linejoin="round" fill="none" d="M595.0 196.0 L595.0 209.0 "/>
+    <path stroke="#000000" stroke-width="1.000000" stroke-linecap="round" stroke-linejoin="round" fill="none" d="M785.0 196.0 L785.0 209.0 "/>
+    <path stroke="#000000" stroke-width="1.000000" stroke-linecap="round" stroke-linejoin="round" fill="none" d="M975.0 196.0 L975.0 209.0 "/>
+    <text x="40" y="68" font-family="Courier" font-size="15" stroke="none" fill="#000000">products</text>
+    <text x="40" y="82" font-family="Courier" font-size="15" stroke="none" fill="#000000">(5, 5.00, [1 Jan 2020,1 Jan 2022))</text>
+    <text x="20" y="222" font-family="Courier" font-size="15" stroke="none" fill="#000000">2020</text>
+    <text x="230" y="138" font-family="Courier" font-size="15" stroke="none" fill="#000000">products</text>
+    <text x="230" y="152" font-family="Courier" font-size="15" stroke="none" fill="#000000">(6, 9.00, [1 Jan 2021,1 Jan 2024))</text>
+    <text x="210" y="222" font-family="Courier" font-size="15" stroke="none" fill="#000000">2021</text>
+    <text x="400" y="222" font-family="Courier" font-size="15" stroke="none" fill="#000000">2022</text>
+    <text x="420" y="68" font-family="Courier" font-size="15" stroke="none" fill="#000000">products</text>
+    <text x="420" y="82" font-family="Courier" font-size="15" stroke="none" fill="#000000">(5, 8.00, [1 Jan 2022,))</text>
+    <text x="590" y="222" font-family="Courier" font-size="15" stroke="none" fill="#000000">2023</text>
+    <text x="780" y="222" font-family="Courier" font-size="15" stroke="none" fill="#000000">2024</text>
+    <text x="979" y="222" font-family="Courier" font-size="15" stroke="none" fill="#000000">...</text>
+  </g>
+</svg>
diff --git a/doc/src/sgml/images/temporal-entities.txt b/doc/src/sgml/images/temporal-entities.txt
new file mode 100644
index 00000000000..0def28e0a59
--- /dev/null
+++ b/doc/src/sgml/images/temporal-entities.txt
@@ -0,0 +1,14 @@
++-------------------------------------+-------------------------------------------------------+
+| cGRE                                | cGRE                                                  |
+| products                            | products                                              |
+| (5, 5.00, [1 Jan 2020,1 Jan 2022))  | (5, 8.00, [1 Jan 2022,))                              |
+|                                     |                                                       |
++------------------+------------------+-------------------------------------+-----------------+
+                   | cGRE                                                   |
+                   | products                                               |
+                   | (6, 9.00, [1 Jan 2021,1 Jan 2024))                     |
+                   |                                                        |
+                   +--------------------------------------------------------+
+
+|                  |                  |                  |                  |                  |
+2020               2021               2022               2023               2024               ...
diff --git a/doc/src/sgml/images/temporal-references.svg b/doc/src/sgml/images/temporal-references.svg
new file mode 100644
index 00000000000..09230c4e4e9
--- /dev/null
+++ b/doc/src/sgml/images/temporal-references.svg
@@ -0,0 +1,37 @@
+<?xml version="1.0"?>
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 1020 322" width="1020" height="322" shape-rendering="geometricPrecision" version="1.0">
+  <defs>
+    <filter id="f2" x="0" y="0" width="200%" height="200%">
+      <feOffset result="offOut" in="SourceGraphic" dx="5" dy="5"/>
+      <feGaussianBlur result="blurOut" in="offOut" stdDeviation="3"/>
+      <feBlend in="SourceGraphic" in2="blurOut" mode="normal"/>
+    </filter>
+  </defs>
+  <g stroke-width="1" stroke-linecap="square" stroke-linejoin="round">
+    <rect x="0" y="0" width="1020" height="322" style="fill: #ffffff"/>
+    <path stroke="#000000" stroke-width="1.000000" stroke-linecap="round" stroke-linejoin="round" fill="#99dd99" d="M395.0 105.0 L395.0 35.0 L945.0 35.0 L945.0 105.0 z"/>
+    <path stroke="#000000" stroke-width="1.000000" stroke-linecap="round" stroke-linejoin="round" fill="#ffff33" d="M215.0 105.0 L215.0 175.0 L685.0 175.0 L685.0 105.0 z"/>
+    <path stroke="#000000" stroke-width="1.000000" stroke-linecap="round" stroke-linejoin="round" fill="#ffff33" d="M455.0 175.0 L875.0 175.0 L875.0 245.0 L455.0 245.0 z"/>
+    <path stroke="#000000" stroke-width="1.000000" stroke-linecap="round" stroke-linejoin="round" fill="#99dd99" d="M25.0 35.0 L25.0 105.0 L395.0 105.0 L395.0 35.0 z"/>
+    <path stroke="#000000" stroke-width="1.000000" stroke-linecap="round" stroke-linejoin="round" fill="none" d="M25.0 266.0 L25.0 279.0 "/>
+    <path stroke="#000000" stroke-width="1.000000" stroke-linecap="round" stroke-linejoin="round" fill="none" d="M405.0 266.0 L405.0 279.0 "/>
+    <path stroke="#000000" stroke-width="1.000000" stroke-linecap="round" stroke-linejoin="round" fill="none" d="M215.0 266.0 L215.0 279.0 "/>
+    <path stroke="#000000" stroke-width="1.000000" stroke-linecap="round" stroke-linejoin="round" fill="none" d="M595.0 266.0 L595.0 279.0 "/>
+    <path stroke="#000000" stroke-width="1.000000" stroke-linecap="round" stroke-linejoin="round" fill="none" d="M785.0 266.0 L785.0 279.0 "/>
+    <path stroke="#000000" stroke-width="1.000000" stroke-linecap="round" stroke-linejoin="round" fill="none" d="M975.0 266.0 L975.0 279.0 "/>
+    <text x="40" y="68" font-family="Courier" font-size="15" stroke="none" fill="#000000">products</text>
+    <text x="40" y="82" font-family="Courier" font-size="15" stroke="none" fill="#000000">(5, 5.00, [1 Jan 2020,1 Jan 2022))</text>
+    <text x="210" y="292" font-family="Courier" font-size="15" stroke="none" fill="#000000">2021</text>
+    <text x="230" y="138" font-family="Courier" font-size="15" stroke="none" fill="#000000">variants</text>
+    <text x="230" y="152" font-family="Courier" font-size="15" stroke="none" fill="#000000">(8, 5, 'Medium', [1 Jan 2021,1 Jun 2023))</text>
+    <text x="20" y="292" font-family="Courier" font-size="15" stroke="none" fill="#000000">2020</text>
+    <text x="470" y="208" font-family="Courier" font-size="15" stroke="none" fill="#000000">variants</text>
+    <text x="470" y="222" font-family="Courier" font-size="15" stroke="none" fill="#000000">(9, 5, 'XXL', [1 Mar 2022,1 Jun 2024))</text>
+    <text x="410" y="68" font-family="Courier" font-size="15" stroke="none" fill="#000000">products</text>
+    <text x="410" y="82" font-family="Courier" font-size="15" stroke="none" fill="#000000">(5, 8.00, [1 Jan 2022,))</text>
+    <text x="590" y="292" font-family="Courier" font-size="15" stroke="none" fill="#000000">2023</text>
+    <text x="400" y="292" font-family="Courier" font-size="15" stroke="none" fill="#000000">2022</text>
+    <text x="780" y="292" font-family="Courier" font-size="15" stroke="none" fill="#000000">2024</text>
+    <text x="979" y="292" font-family="Courier" font-size="15" stroke="none" fill="#000000">...</text>
+  </g>
+</svg>
diff --git a/doc/src/sgml/images/temporal-references.txt b/doc/src/sgml/images/temporal-references.txt
new file mode 100644
index 00000000000..57dedc32e0b
--- /dev/null
+++ b/doc/src/sgml/images/temporal-references.txt
@@ -0,0 +1,19 @@
++------------------------------------+------------------------------------------------------+
+| cGRE                               | cGRE                                                 |
+| products                           | products                                             |
+| (5, 5.00, [1 Jan 2020,1 Jan 2022)) | (5, 8.00, [1 Jan 2022,))                             |
+|                                    |                                                      |
++------------------+-----------------+----------------------------+-------------------------+
+                   | cYEL                                         |
+                   | variants                                     |
+                   | (8, 5, 'Medium', [1 Jan 2021,1 Jun 2023))    |
+                   |                                              |
+                   +-----------------------+----------------------+------------------+
+                                           | cYEL                                    |
+                                           | variants                                |
+                                           | (9, 5, 'XXL', [1 Mar 2022,1 Jun 2024))  |
+                                           |                                         |
+                                           +-----------------------------------------+
+
+|                  |                  |                  |                  |                  |
+2020               2021               2022               2023               2024               ...