Make extract() do something more reasonable with infinite datetimes.

Historically, extract() just returned zero for any case involving an
infinite timestamp[tz] input; even cases in which the unit name was
invalid.  This is not very sensible.  Instead, return infinity or
-infinity as appropriate when the requested field is one that is
monotonically increasing (e.g, year, epoch), or NULL when it is not
(e.g., day, hour).  Also, throw the expected errors for bad unit names.

BACKWARDS INCOMPATIBLE CHANGE

Vitaly Burovoy, reviewed by Vik Fearing

Author: tglsfdc

doc/src/sgml/func.sgml

@@ -7521,6 +7521,17 @@ SELECT EXTRACT(YEAR FROM TIMESTAMP '2001-02-16 20:38:40');
     </variablelist>
    </para>
 
+   <note>
+    <para>
+     When the input value is +/-Infinity, <function>extract</> returns
+     +/-Infinity for monotonically-increasing fields (<literal>epoch</>,
+     <literal>julian</>, <literal>year</>, <literal>isoyear</>,
+     <literal>decade</>, <literal>century</>, and <literal>millennium</>).
+     For other fields, NULL is returned.  <productname>PostgreSQL</>
+     versions before 9.6 returned zero for all cases of infinite input.
+    </para>
+   </note>
+
    <para>
     The <function>extract</function> function is primarily intended
     for computational processing.  For formatting date/time values for

src/backend/utils/adt/timestamp.c

@@ -4311,6 +4311,83 @@ date2isoyearday(int year, int mon, int mday)
 	return date2j(year, mon, mday) - isoweek2j(date2isoyear(year, mon, mday), 1) + 1;
 }
 
+/*
+ * NonFiniteTimestampTzPart
+ *
+ *	Used by timestamp_part and timestamptz_part when extracting from infinite
+ *	timestamp[tz].  Returns +/-Infinity if that is the appropriate result,
+ *	otherwise returns zero (which should be taken as meaning to return NULL).
+ *
+ *	Errors thrown here for invalid units should exactly match those that
+ *	would be thrown in the calling functions, else there will be unexpected
+ *	discrepancies between finite- and infinite-input cases.
+ */
+static float8
+NonFiniteTimestampTzPart(int type, int unit, char *lowunits,
+						 bool isNegative, bool isTz)
+{
+	if ((type != UNITS) && (type != RESERV))
+	{
+		if (isTz)
+			ereport(ERROR,
+					(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+			   errmsg("timestamp with time zone units \"%s\" not recognized",
+					  lowunits)));
+		else
+			ereport(ERROR,
+					(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+					 errmsg("timestamp units \"%s\" not recognized",
+							lowunits)));
+	}
+
+	switch (unit)
+	{
+			/* Oscillating units */
+		case DTK_MICROSEC:
+		case DTK_MILLISEC:
+		case DTK_SECOND:
+		case DTK_MINUTE:
+		case DTK_HOUR:
+		case DTK_DAY:
+		case DTK_MONTH:
+		case DTK_QUARTER:
+		case DTK_WEEK:
+		case DTK_DOW:
+		case DTK_ISODOW:
+		case DTK_DOY:
+		case DTK_TZ:
+		case DTK_TZ_MINUTE:
+		case DTK_TZ_HOUR:
+			return 0.0;
+
+			/* Monotonically-increasing units */
+		case DTK_YEAR:
+		case DTK_DECADE:
+		case DTK_CENTURY:
+		case DTK_MILLENNIUM:
+		case DTK_JULIAN:
+		case DTK_ISOYEAR:
+		case DTK_EPOCH:
+			if (isNegative)
+				return -get_float8_infinity();
+			else
+				return get_float8_infinity();
+
+		default:
+			if (isTz)
+				ereport(ERROR,
+						(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				errmsg("timestamp with time zone units \"%s\" not supported",
+					   lowunits)));
+			else
+				ereport(ERROR,
+						(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+						 errmsg("timestamp units \"%s\" not supported",
+								lowunits)));
+			return 0.0;			/* keep compiler quiet */
+	}
+}
+
 /* timestamp_part()
  * Extract specified field from timestamp.
  */
@@ -4327,12 +4404,6 @@ timestamp_part(PG_FUNCTION_ARGS)
 	struct pg_tm tt,
 			   *tm = &tt;
 
-	if (TIMESTAMP_NOT_FINITE(timestamp))
-	{
-		result = 0;
-		PG_RETURN_FLOAT8(result);
-	}
-
 	lowunits = downcase_truncate_identifier(VARDATA_ANY(units),
 											VARSIZE_ANY_EXHDR(units),
 											false);
@@ -4341,6 +4412,17 @@ timestamp_part(PG_FUNCTION_ARGS)
 	if (type == UNKNOWN_FIELD)
 		type = DecodeSpecial(0, lowunits, &val);
 
+	if (TIMESTAMP_NOT_FINITE(timestamp))
+	{
+		result = NonFiniteTimestampTzPart(type, val, lowunits,
+										  TIMESTAMP_IS_NOBEGIN(timestamp),
+										  false);
+		if (result)
+			PG_RETURN_FLOAT8(result);
+		else
+			PG_RETURN_NULL();
+	}
+
 	if (type == UNITS)
 	{
 		if (timestamp2tm(timestamp, NULL, tm, &fsec, NULL, NULL) != 0)
@@ -4538,12 +4620,6 @@ timestamptz_part(PG_FUNCTION_ARGS)
 	struct pg_tm tt,
 			   *tm = &tt;
 
-	if (TIMESTAMP_NOT_FINITE(timestamp))
-	{
-		result = 0;
-		PG_RETURN_FLOAT8(result);
-	}
-
 	lowunits = downcase_truncate_identifier(VARDATA_ANY(units),
 											VARSIZE_ANY_EXHDR(units),
 											false);
@@ -4552,6 +4628,17 @@ timestamptz_part(PG_FUNCTION_ARGS)
 	if (type == UNKNOWN_FIELD)
 		type = DecodeSpecial(0, lowunits, &val);
 
+	if (TIMESTAMP_NOT_FINITE(timestamp))
+	{
+		result = NonFiniteTimestampTzPart(type, val, lowunits,
+										  TIMESTAMP_IS_NOBEGIN(timestamp),
+										  true);
+		if (result)
+			PG_RETURN_FLOAT8(result);
+		else
+			PG_RETURN_NULL();
+	}
+
 	if (type == UNITS)
 	{
 		if (timestamp2tm(timestamp, &tz, tm, &fsec, NULL, NULL) != 0)