Revert use singular for -1 (commits 9ee7d533 and 5da9868e

Turns out you can specify negative values using plurals:

	https://english.stackexchange.com/questions/9735/is-1-followed-by-a-singular-or-plural-noun

so the previous code was correct enough, and consistent with other usage
in our code.  Also add comment in the two places where this could be
confused.

Reported-by: Noah Misch

Diagnosed-by: 20210425115726.GA2353095@rfd.leadboat.com

contrib/dblink/expected/dblink.out

@@ -1100,8 +1100,8 @@ FROM dblink('myconn',
     'SELECT * FROM (VALUES (''-1 2:03:04'')) i')
   AS i(i interval);
          i         
-------------------
- -1 day -02:03:04
+-------------------
+ -1 days -02:03:04
 (1 row)
 
 -- Try swapping to another format to ensure the GUCs are tracked

src/backend/utils/adt/datetime.c

@@ -4190,7 +4190,7 @@ AddPostgresIntPart(char *cp, int value, const char *units,
 			(*is_before && value > 0) ? "+" : "",
 			value,
 			units,
-			(abs(value) != 1) ? "s" : "");
+			(value != 1) ? "s" : "");
 
 	/*
 	 * Each nonzero field sets is_before for (only) the next one.  This is a
@@ -4216,7 +4216,7 @@ AddVerboseIntPart(char *cp, int value, const char *units,
 	}
 	else if (*is_before)
 		value = -value;
-	sprintf(cp, " %d %s%s", value, units, (abs(value) == 1) ? "" : "s");
+	sprintf(cp, " %d %s%s", value, units, (value == 1) ? "" : "s");
 	*is_zero = false;
 	return cp + strlen(cp);
 }
@@ -4414,6 +4414,7 @@ EncodeInterval(struct pg_tm *tm, fsec_t fsec, int style, char *str)
 				else if (is_before)
 					*cp++ = '-';
 				cp = AppendSeconds(cp, sec, fsec, MAX_INTERVAL_PRECISION, false);
+				/* We output "ago", not negatives, so use abs(). */
 				sprintf(cp, " sec%s",
 						(abs(sec) != 1 || fsec != 0) ? "s" : "");
 				is_zero = false;

src/interfaces/ecpg/pgtypeslib/interval.c

@@ -694,7 +694,7 @@ AddVerboseIntPart(char *cp, int value, const char *units,
 	}
 	else if (*is_before)
 		value = -value;
-	sprintf(cp, " %d %s%s", value, units, (abs(value) == 1) ? "" : "s");
+	sprintf(cp, " %d %s%s", value, units, (value == 1) ? "" : "s");
 	*is_zero = false;
 	return cp + strlen(cp);
 }
@@ -711,7 +711,7 @@ AddPostgresIntPart(char *cp, int value, const char *units,
 			(*is_before && value > 0) ? "+" : "",
 			value,
 			units,
-			(abs(value) != 1) ? "s" : "");
+			(value != 1) ? "s" : "");
 
 	/*
 	 * Each nonzero field sets is_before for (only) the next one.  This is a
@@ -924,6 +924,7 @@ EncodeInterval(struct /* pg_ */ tm *tm, fsec_t fsec, int style, char *str)
 					*cp++ = '-';
 				AppendSeconds(cp, sec, fsec, MAX_INTERVAL_PRECISION, false);
 				cp += strlen(cp);
+				/* We output "ago", not negatives, so use abs(). */
 				sprintf(cp, " sec%s",
 						(abs(sec) != 1 || fsec != 0) ? "s" : "");
 				is_zero = false;

src/interfaces/libpq/fe-print.c

@@ -303,7 +303,7 @@ PQprint(FILE *fout, const PGresult *res, const PQprintOpt *po)
 		}
 		if (po->header && !po->html3)
 			fprintf(fout, "(%d row%s)\n\n", PQntuples(res),
-					(abs(PQntuples(res)) == 1) ? "" : "s");
+					(PQntuples(res) == 1) ? "" : "s");
 		if (po->html3 && !po->expanded)
 			fputs("</table>\n", fout);
 		free(fieldMax);
@@ -662,7 +662,7 @@ PQdisplayTuples(const PGresult *res,
 
 	if (!quiet)
 		fprintf(fp, "\nQuery returned %d row%s.\n", PQntuples(res),
-				(abs(PQntuples(res)) == 1) ? "" : "s");
+				(PQntuples(res) == 1) ? "" : "s");
 
 	fflush(fp);
 

src/test/regress/expected/interval.out

@@ -24,14 +24,14 @@ SELECT INTERVAL '-08:00' AS "Eight hours";
 
 SELECT INTERVAL '-1 +02:03' AS "22 hours ago...";
   22 hours ago...  
-------------------
- -1 day +02:03:00
+-------------------
+ -1 days +02:03:00
 (1 row)
 
 SELECT INTERVAL '-1 days +02:03' AS "22 hours ago...";
   22 hours ago...  
-------------------
- -1 day +02:03:00
+-------------------
+ -1 days +02:03:00
 (1 row)
 
 SELECT INTERVAL '1.5 weeks' AS "Ten days twelve hours";
@@ -288,7 +288,7 @@ FROM INTERVAL_MULDIV_TBL;
               product               
 ------------------------------------
  1 year 12 days 122:24:00
- -1 year -12 days +93:36:00
+ -1 years -12 days +93:36:00
  -3 days -14:24:00
  2 mons 13 days 01:22:28.8
  -10 mons +120 days 37:28:21.6567
@@ -317,7 +317,7 @@ FROM INTERVAL_MULDIV_TBL;
 ----------------------------------
  4 mons 4 days 40:48:00
  -4 mons -4 days +31:12:00
- -1 day -04:48:00
+ -1 days -04:48:00
  25 days -15:32:30.4
  -3 mons +30 days 12:29:27.2189
  12 days
@@ -786,8 +786,8 @@ SELECT  interval '+1 -1:00:00',
         interval '+1-2 -3 +4:05:06.789',
         interval '-1-2 +3 -4:05:06.789';
     interval     |     interval      |              interval               |                interval                
------------------+------------------+-------------------------------------+---------------------------------------
- 1 day -01:00:00 | -1 day +01:00:00 | 1 year 2 mons -3 days +04:05:06.789 | -1 year -2 mons +3 days -04:05:06.789
+-----------------+-------------------+-------------------------------------+----------------------------------------
+ 1 day -01:00:00 | -1 days +01:00:00 | 1 year 2 mons -3 days +04:05:06.789 | -1 years -2 mons +3 days -04:05:06.789
 (1 row)
 
 -- test output of couple non-standard interval values in the sql style