Add temporal PRIMARY KEY and UNIQUE constraints

Add WITHOUT OVERLAPS clause to PRIMARY KEY and UNIQUE constraints.
These are backed by GiST indexes instead of B-tree indexes, since they
are essentially exclusion constraints with = for the scalar parts of
the key and && for the temporal part.

(previously committed as 46a0cd4cefb, reverted by 46a0cd4cefb; the new
part is this:)

Because 'empty' && 'empty' is false, the temporal PK/UQ constraint
allowed duplicates, which is confusing to users and breaks internal
expectations.  For instance, when GROUP BY checks functional
dependencies on the PK, it allows selecting other columns from the
table, but in the presence of duplicate keys you could get the value
from any of their rows.  So we need to forbid empties.

This all means that at the moment we can only support ranges and
multiranges for temporal PK/UQs, unlike the original patch (above).
Documentation and tests for this are added.  But this could
conceivably be extended by introducing some more general support for
the notion of "empty" for other types.

Author: Paul A. Jungwirth <pj@illuminatedcomputing.com>
Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Reviewed-by: jian he <jian.universality@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/CA+renyUApHgSZF9-nd-a0+OPGharLQLO=mDHcY4_qQ0+noCUVg@mail.gmail.com

1 files changed, 10 insertions, 1 deletions

diff --git a/doc/src/sgml/gist.sgml b/doc/src/sgml/gist.sgml
index f789824c83b..638d912dc2d 100644
--- a/doc/src/sgml/gist.sgml
+++ b/doc/src/sgml/gist.sgml
@@ -292,7 +292,8 @@ CREATE INDEX ON my_table USING GIST (my_inet_column inet_ops);
    The optional twelfth method <function>stratnum</function> is used to
    translate well-known <literal>RT*StrategyNumber</literal>s (from
    <filename>src/include/access/stratnum.h</filename>) into strategy numbers
-   used by the operator class.
+   used by the operator class.  This lets the core code look up operators for
+   temporal constraint indexes.
  </para>
 
  <variablelist>
@@ -1180,6 +1181,14 @@ my_sortsupport(PG_FUNCTION_ARGS)
       </para>
 
       <para>
+       This is used for temporal index constraints (i.e., <literal>PRIMARY
+       KEY</literal> and <literal>UNIQUE</literal>).  If the operator class
+       provides this function and it returns results for
+       <literal>RTEqualStrategyNumber</literal>, it can be used in the
+       non-<literal>WITHOUT OVERLAPS</literal> part(s) of an index constraint.
+      </para>
+
+      <para>
        The <acronym>SQL</acronym> declaration of the function must look like
        this: