Change return type of EXTRACT to numeric

The previous implementation of EXTRACT mapped internally to
date_part(), which returned type double precision (since it was
implemented long before the numeric type existed).  This can lead to
imprecise output in some cases, so returning numeric would be
preferrable.  Changing the return type of an existing function is a
bit risky, so instead we do the following:  We implement a new set of
functions, which are now called "extract", in parallel to the existing
date_part functions.  They work the same way internally but use
numeric instead of float8.  The EXTRACT construct is now mapped by the
parser to these new extract functions.  That way, dumps of views
etc. from old versions (which would use date_part) continue to work
unchanged, but new uses will map to the new extract functions.

Additionally, the reverse compilation of EXTRACT now reproduces the
original syntax, using the new mechanism introduced in
40c24bfef9.

The following minor changes of behavior result from the new
implementation:

- The column name from an isolated EXTRACT call is now "extract"
  instead of "date_part".

- Extract from date now rejects inappropriate field names such as
  HOUR.  It was previously mapped internally to extract from
  timestamp, so it would silently accept everything appropriate for
  timestamp.

- Return values when extracting fields with possibly fractional
  values, such as second and epoch, now have the full scale that the
  value has internally (so, for example, '1.000000' instead of just
  '1').

Reported-by: Petr Fedorov <petr.fedorov@phystech.edu>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/42b73d2d-da12-ba9f-570a-420e0cce19d9@phystech.edu

doc/src/sgml/func.sgml

@@ -8872,7 +8872,7 @@ SELECT regexp_match('abc01234xyz', '(?:(.*?)(\d+)(.*)){1,1}');
           <primary>extract</primary>
          </indexterm>
          <function>extract</function> ( <parameter>field</parameter> <literal>from</literal> <type>timestamp</type> )
-         <returnvalue>double precision</returnvalue>
+         <returnvalue>numeric</returnvalue>
         </para>
         <para>
          Get timestamp subfield; see <xref linkend="functions-datetime-extract"/>
@@ -8886,7 +8886,7 @@ SELECT regexp_match('abc01234xyz', '(?:(.*?)(\d+)(.*)){1,1}');
        <row>
         <entry role="func_table_entry"><para role="func_signature">
          <function>extract</function> ( <parameter>field</parameter> <literal>from</literal> <type>interval</type> )
-         <returnvalue>double precision</returnvalue>
+         <returnvalue>numeric</returnvalue>
         </para>
         <para>
          Get interval subfield; see <xref linkend="functions-datetime-extract"/>
@@ -9401,7 +9401,7 @@ EXTRACT(<replaceable>field</replaceable> FROM <replaceable>source</replaceable>)
     well.)  <replaceable>field</replaceable> is an identifier or
     string that selects what field to extract from the source value.
     The <function>extract</function> function returns values of type
-    <type>double precision</type>.
+    <type>numeric</type>.
     The following are valid field names:
 
     <!-- alphabetical -->
@@ -9825,6 +9825,10 @@ date_part('<replaceable>field</replaceable>', <replaceable>source</replaceable>)
     be a string value, not a name.  The valid field names for
     <function>date_part</function> are the same as for
     <function>extract</function>.
+    For historical reasons, the <function>date_part</function> function
+    returns values of type <type>double precision</type>.  This can result in
+    a loss of precision in certain uses.  Using <function>extract</function>
+    is recommended instead.
    </para>
 
 <screen>