Document impact of datestyle on jsonpath string()

Add tests demonstrating the output of the jsonpath `string()` method for
the different date and time data types, and how the `datestyle` GUC
determines that output. Note this relationship in the documentation for
the `string()` method, as well.

Author: theory

doc/src/sgml/func.sgml

@@ -17971,7 +17971,9 @@ ERROR:  jsonpath member accessor can only be applied to an object
         <returnvalue><replaceable>string</replaceable></returnvalue>
        </para>
        <para>
-        String value converted from a JSON boolean, number, string, or datetime
+        String value converted from a JSON boolean, number, string, or
+        datetime. Note that the string output of datetimes is determined by
+        the <xref linkend="guc-datestyle"/> parameter.
        </para>
        <para>
         <literal>jsonb_path_query_array('[1.23, "xyz", false]', '$[*].string()')</literal>

src/test/regress/expected/jsonb_jsonpath.out

@@ -2657,6 +2657,95 @@ select jsonb_path_query_array('[1.23, "yes", false]', '$[*].string().type()');
  ["string", "string", "string"]
 (1 row)
 
+select jsonb_path_query('"2023-08-15 12:34:56"', '$.timestamp_tz().string()');
+ERROR:  cannot convert value from timestamp to timestamptz without time zone usage
+HINT:  Use *_tz() function for time zone support.
+select jsonb_path_query_tz('"2023-08-15 12:34:56"', '$.timestamp_tz().string()'); -- should work
+      jsonb_path_query_tz       
+--------------------------------
+ "Tue Aug 15 12:34:56 2023 PDT"
+(1 row)
+
+select jsonb_path_query('"2023-08-15 12:34:56 +5:30"', '$.timestamp_tz().string()');
+        jsonb_path_query        
+--------------------------------
+ "Tue Aug 15 00:04:56 2023 PDT"
+(1 row)
+
+select jsonb_path_query('"2023-08-15 12:34:56"', '$.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()'); -- should work
+ 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"', '$.timestamp_tz().string()'); -- should work
+   jsonb_path_query_tz    
+--------------------------
+ "2023-08-15 12:34:56-07"
+(1 row)
+
+select jsonb_path_query('"2023-08-15 12:34:56 +5:30"', '$.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"', '$.timestamp().string()');
+   jsonb_path_query    
+-----------------------
+ "2023-08-15 12:34:56"
+(1 row)
+
+select jsonb_path_query('"12:34:56 +5:30"', '$.time_tz().string()');
+ jsonb_path_query 
+------------------
+ "12:34:56+05:30"
+(1 row)
+
+select jsonb_path_query_tz('"12:34:56"', '$.time_tz().string()'); -- should work
+ jsonb_path_query_tz 
+---------------------
+ "12:34:56-07"
+(1 row)
+
+select jsonb_path_query('"12:34:56"', '$.time().string()');
+ jsonb_path_query 
+------------------
+ "12:34:56"
+(1 row)
+
+select jsonb_path_query('"2023-08-15"', '$.date().string()');
+ jsonb_path_query 
+------------------
+ "2023-08-15"
+(1 row)
+
+reset datestyle;
 -- Test .time()
 select jsonb_path_query('null', '$.time()');
 ERROR:  jsonpath item method .time() can only be applied to a string

src/test/regress/sql/jsonb_jsonpath.sql

@@ -602,6 +602,24 @@ select jsonb_path_query('"2023-08-15 12:34:56 +5:30"', '$.timestamp().string()')
 select jsonb_path_query_tz('"2023-08-15 12:34:56 +5:30"', '$.timestamp().string()'); -- should work
 select jsonb_path_query_array('[1.23, "yes", false]', '$[*].string()');
 select jsonb_path_query_array('[1.23, "yes", false]', '$[*].string().type()');
+select jsonb_path_query('"2023-08-15 12:34:56"', '$.timestamp_tz().string()');
+select jsonb_path_query_tz('"2023-08-15 12:34:56"', '$.timestamp_tz().string()'); -- should work
+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"', '$.time_tz().string()');
+select jsonb_path_query_tz('"12:34:56"', '$.time_tz().string()'); -- should work
+select jsonb_path_query('"12:34:56"', '$.time().string()');
+select jsonb_path_query('"2023-08-15"', '$.date().string()');
+
+set datestyle = 'ISO';
+select jsonb_path_query_tz('"2023-08-15 12:34:56"', '$.timestamp_tz().string()'); -- should work
+select jsonb_path_query('"2023-08-15 12:34:56 +5:30"', '$.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()');
+select jsonb_path_query_tz('"12:34:56"', '$.time_tz().string()'); -- should work
+select jsonb_path_query('"12:34:56"', '$.time().string()');
+select jsonb_path_query('"2023-08-15"', '$.date().string()');
+reset datestyle;
 
 -- Test .time()
 select jsonb_path_query('null', '$.time()');