Make jsonpath .string() be immutable for datetimes.

Discussion of commit ed055d249 revealed that we don't actually
want jsonpath's .string() method to depend on DateStyle, nor
TimeZone either, because the non-"_tz" jsonpath functions are
supposed to be immutable.  Potentially we could allow a TimeZone
dependency in the "_tz" variants, but it seems better to just
uniformly define this method as returning the same string that
jsonb text output would do.  That's easier to implement too,
saving a couple dozen lines.

Patch by me, per complaint from Peter Eisentraut.  Back-patch
to v17 where this feature came in (in 66ea94e8e).  Also
back-patch ed055d249 to provide test cases.

Discussion: https://postgr.es/m/5e8879d0-a3c8-4be2-950f-d83aa2af953a@eisentraut.org

doc/src/sgml/func.sgml

@@ -18017,16 +18017,15 @@ ERROR:  jsonpath member accessor can only be applied to an object
        </para>
        <para>
         String value converted from a JSON boolean, number, string, or
-        datetime (the output format for datetimes is determined by
+        datetime
-        the <xref linkend="guc-datestyle"/> parameter)
        </para>
        <para>
         <literal>jsonb_path_query_array('[1.23, "xyz", false]', '$[*].string()')</literal>
         <returnvalue>["1.23", "xyz", "false"]</returnvalue>
        </para>
        <para>
-        <literal>jsonb_path_query('"2023-08-15"', '$.datetime().string()')</literal>
+        <literal>jsonb_path_query('"2023-08-15 12:34:56"', '$.timestamp().string()')</literal>
-        <returnvalue>"2023-08-15"</returnvalue>
+        <returnvalue>"2023-08-15T12:34:56"</returnvalue>
        </para></entry>
       </row>
 

src/backend/utils/adt/jsonpath_exec.c

@@ -72,6 +72,7 @@
 #include "utils/datetime.h"
 #include "utils/float.h"
 #include "utils/formatting.h"
+#include "utils/json.h"
 #include "utils/jsonpath.h"
 #include "utils/lsyscache.h"
 #include "utils/memutils.h"
@@ -1629,32 +1630,13 @@ executeItemOptUnwrapTarget(JsonPathExecContext *cxt, JsonPathItem *jsp,
 						break;
 					case jbvDatetime:
 						{
-							switch (jb->val.datetime.typid)
+							char		buf[MAXDATELEN + 1];
-							{
+
-								case DATEOID:
+							JsonEncodeDateTime(buf,
-									tmp = DatumGetCString(DirectFunctionCall1(date_out,
+											   jb->val.datetime.value,
-																			  jb->val.datetime.value));
+											   jb->val.datetime.typid,
-									break;
+											   &jb->val.datetime.tz);
-								case TIMEOID:
+							tmp = pstrdup(buf);
-									tmp = DatumGetCString(DirectFunctionCall1(time_out,
-																			  jb->val.datetime.value));
-									break;
-								case TIMETZOID:
-									tmp = DatumGetCString(DirectFunctionCall1(timetz_out,
-																			  jb->val.datetime.value));
-									break;
-								case TIMESTAMPOID:
-									tmp = DatumGetCString(DirectFunctionCall1(timestamp_out,
-																			  jb->val.datetime.value));
-									break;
-								case TIMESTAMPTZOID:
-									tmp = DatumGetCString(DirectFunctionCall1(timestamptz_out,
-																			  jb->val.datetime.value));
-									break;
-								default:
-									elog(ERROR, "unrecognized SQL/JSON datetime type oid: %u",
-										 jb->val.datetime.typid);
-							}
 						}
 						break;
 					case jbvNull:

src/test/regress/expected/jsonb_jsonpath.out

@@ -2653,8 +2653,8 @@ ERROR:  cannot convert value from timestamptz to timestamp without time zone usa
 HINT:  Use *_tz() function for time zone support.
 select jsonb_path_query_tz('"2023-08-15 12:34:56 +5:30"', '$.timestamp().string()'); -- should work
   jsonb_path_query_tz  
-----------------------------
+-----------------------
- "Tue Aug 15 00:04:56 2023"
+ "2023-08-15T00:04:56"
 (1 row)
 
 select jsonb_path_query('"2023-08-15 12:34:56"', '$.timestamp_tz().string()');
@@ -2662,63 +2662,20 @@ ERROR:  cannot convert value from timestamp to timestamptz without time zone usa
 HINT:  Use *_tz() function for time zone support.
 select jsonb_path_query_tz('"2023-08-15 12:34:56"', '$.timestamp_tz().string()'); -- should work
      jsonb_path_query_tz     
---------------------------------
+-----------------------------
- "Tue Aug 15 12:34:56 2023 PDT"
+ "2023-08-15T12:34:56-07:00"
 (1 row)
 
 select jsonb_path_query('"2023-08-15 12:34:56 +5:30"', '$.timestamp_tz().string()');
       jsonb_path_query       
---------------------------------
+-----------------------------
- "Tue Aug 15 00:04:56 2023 PDT"
+ "2023-08-15T12:34:56+05:30"
-(1 row)
-
-select jsonb_path_query('"2023-08-15 12:34:56"', '$.timestamp().string()');
-      jsonb_path_query      
-----------------------------
- "Tue Aug 15 12:34:56 2023"
-(1 row)
-
-select jsonb_path_query('"12:34:56 +5:30"', '$.time_tz().string()');
- jsonb_path_query 
-------------------
- "12:34:56+05:30"
-(1 row)
-
-select jsonb_path_query_tz('"12:34:56"', '$.time_tz().string()');
- jsonb_path_query_tz 
----------------------
- "12:34:56-07"
-(1 row)
-
-select jsonb_path_query('"12:34:56"', '$.time().string()');
- jsonb_path_query 
-------------------
- "12:34:56"
-(1 row)
-
-select jsonb_path_query('"2023-08-15"', '$.date().string()');
- jsonb_path_query 
-------------------
- "08-15-2023"
-(1 row)
-
-set datestyle = 'ISO';
-select jsonb_path_query_tz('"2023-08-15 12:34:56"', '$.timestamp_tz().string()');
-   jsonb_path_query_tz    
---------------------------
- "2023-08-15 12:34:56-07"
-(1 row)
-
-select jsonb_path_query('"2023-08-15 12:34:56 +5:30"', '$.timestamp_tz().string()');
-     jsonb_path_query     
---------------------------
- "2023-08-15 00:04:56-07"
 (1 row)
 
 select jsonb_path_query('"2023-08-15 12:34:56"', '$.timestamp().string()');
    jsonb_path_query    
 -----------------------
- "2023-08-15 12:34:56"
+ "2023-08-15T12:34:56"
 (1 row)
 
 select jsonb_path_query('"12:34:56 +5:30"', '$.time_tz().string()');
@@ -2730,7 +2687,7 @@ select jsonb_path_query('"12:34:56 +5:30"', '$.time_tz().string()');
 select jsonb_path_query_tz('"12:34:56"', '$.time_tz().string()');
  jsonb_path_query_tz 
 ---------------------
- "12:34:56-07"
+ "12:34:56-07:00"
 (1 row)
 
 select jsonb_path_query('"12:34:56"', '$.time().string()');
@@ -2745,7 +2702,23 @@ select jsonb_path_query('"2023-08-15"', '$.date().string()');
  "2023-08-15"
 (1 row)
 
-reset datestyle;
+-- .string() does not react to timezone or datestyle
+begin;
+set local timezone = 'UTC';
+set local datestyle = 'German';
+select jsonb_path_query('"2023-08-15 12:34:56 +5:30"', '$.timestamp_tz().string()');
+      jsonb_path_query       
+-----------------------------
+ "2023-08-15T12:34:56+05:30"
+(1 row)
+
+select jsonb_path_query('"2023-08-15 12:34:56"', '$.timestamp().string()');
+   jsonb_path_query    
+-----------------------
+ "2023-08-15T12:34:56"
+(1 row)
+
+rollback;
 -- Test .time()
 select jsonb_path_query('null', '$.time()');
 ERROR:  jsonpath item method .time() can only be applied to a string

src/test/regress/sql/jsonb_jsonpath.sql

@@ -611,15 +611,13 @@ select jsonb_path_query_tz('"12:34:56"', '$.time_tz().string()');
 select jsonb_path_query('"12:34:56"', '$.time().string()');
 select jsonb_path_query('"2023-08-15"', '$.date().string()');
 
-set datestyle = 'ISO';
+-- .string() does not react to timezone or datestyle
-select jsonb_path_query_tz('"2023-08-15 12:34:56"', '$.timestamp_tz().string()');
+begin;
+set local timezone = 'UTC';
+set local datestyle = 'German';
 select jsonb_path_query('"2023-08-15 12:34:56 +5:30"', '$.timestamp_tz().string()');
 select jsonb_path_query('"2023-08-15 12:34:56"', '$.timestamp().string()');
-select jsonb_path_query('"12:34:56 +5:30"', '$.time_tz().string()');
+rollback;
-select jsonb_path_query_tz('"12:34:56"', '$.time_tz().string()');
-select jsonb_path_query('"12:34:56"', '$.time().string()');
-select jsonb_path_query('"2023-08-15"', '$.date().string()');
-reset datestyle;
 
 -- Test .time()
 select jsonb_path_query('null', '$.time()');