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
40c24bfef92530bd846e111c1742c2a54441c62c.

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

1 files changed, 21 insertions, 0 deletions

diff --git a/src/backend/utils/adt/ruleutils.c b/src/backend/utils/adt/ruleutils.c
index 254e8f30501..0b5314e49b3 100644
--- a/src/backend/utils/adt/ruleutils.c
+++ b/src/backend/utils/adt/ruleutils.c
@@ -9782,6 +9782,27 @@ get_func_sql_syntax(FuncExpr *expr, deparse_context *context)
 			appendStringInfoString(buf, "))");
 			return true;
 
+		case F_EXTRACT_TEXT_DATE:
+		case F_EXTRACT_TEXT_TIME:
+		case F_EXTRACT_TEXT_TIMETZ:
+		case F_EXTRACT_TEXT_TIMESTAMP:
+		case F_EXTRACT_TEXT_TIMESTAMPTZ:
+		case F_EXTRACT_TEXT_INTERVAL:
+			/* EXTRACT (x FROM y) */
+			appendStringInfoString(buf, "EXTRACT(");
+			{
+				Const	   *con = (Const *) linitial(expr->args);
+
+				Assert(IsA(con, Const) &&
+					   con->consttype == TEXTOID &&
+					   !con->constisnull);
+				appendStringInfoString(buf, TextDatumGetCString(con->constvalue));
+			}
+			appendStringInfoString(buf, " FROM ");
+			get_rule_expr((Node *) lsecond(expr->args), context, false);
+			appendStringInfoChar(buf, ')');
+			return true;
+
 		case F_IS_NORMALIZED:
 			/* IS xxx NORMALIZED */
 			appendStringInfoString(buf, "((");