JSON generation improvements.

This adds the following:

    json_agg(anyrecord) -> json
    to_json(any) -> json
    hstore_to_json(hstore) -> json (also used as a cast)
    hstore_to_json_loose(hstore) -> json

The last provides heuristic treatment of numbers and booleans.

Also, in json generation, if any non-builtin type has a cast to json,
that function is used instead of the type's output function.

Andrew Dunstan, reviewed by Steve Singer.

Catalog version bumped.

contrib/hstore/expected/hstore.out

@@ -1453,3 +1453,39 @@ select count(*) from testhstore where h = 'pos=>98, line=>371, node=>CBA, indexe
      1
 (1 row)
 
+-- json
+select hstore_to_json('"a key" =>1, b => t, c => null, d=> 12345, e => 012345, f=> 1.234, g=> 2.345e+4');
+                                         hstore_to_json                                          
+-------------------------------------------------------------------------------------------------
+ {"b": "t", "c": null, "d": "12345", "e": "012345", "f": "1.234", "g": "2.345e+4", "a key": "1"}
+(1 row)
+
+select cast( hstore  '"a key" =>1, b => t, c => null, d=> 12345, e => 012345, f=> 1.234, g=> 2.345e+4' as json);
+                                              json                                               
+-------------------------------------------------------------------------------------------------
+ {"b": "t", "c": null, "d": "12345", "e": "012345", "f": "1.234", "g": "2.345e+4", "a key": "1"}
+(1 row)
+
+select hstore_to_json_loose('"a key" =>1, b => t, c => null, d=> 12345, e => 012345, f=> 1.234, g=> 2.345e+4');
+                                   hstore_to_json_loose                                   
+------------------------------------------------------------------------------------------
+ {"b": true, "c": null, "d": 12345, "e": "012345", "f": 1.234, "g": 2.345e+4, "a key": 1}
+(1 row)
+
+create table test_json_agg (f1 text, f2 hstore);
+insert into test_json_agg values ('rec1','"a key" =>1, b => t, c => null, d=> 12345, e => 012345, f=> 1.234, g=> 2.345e+4'),
+       ('rec2','"a key" =>2, b => f, c => "null", d=> -12345, e => 012345.6, f=> -1.234, g=> 0.345e-4');
+select json_agg(q) from test_json_agg q;
+                                                          json_agg                                                          
+----------------------------------------------------------------------------------------------------------------------------
+ [{"f1":"rec1","f2":{"b": "t", "c": null, "d": "12345", "e": "012345", "f": "1.234", "g": "2.345e+4", "a key": "1"}},      +
+  {"f1":"rec2","f2":{"b": "f", "c": "null", "d": "-12345", "e": "012345.6", "f": "-1.234", "g": "0.345e-4", "a key": "2"}}]
+(1 row)
+
+select json_agg(q) from (select f1, hstore_to_json_loose(f2) as f2 from test_json_agg) q;
+                                                       json_agg                                                       
+----------------------------------------------------------------------------------------------------------------------
+ [{"f1":"rec1","f2":{"b": true, "c": null, "d": 12345, "e": "012345", "f": 1.234, "g": 2.345e+4, "a key": 1}},       +
+  {"f1":"rec2","f2":{"b": false, "c": "null", "d": -12345, "e": "012345.6", "f": -1.234, "g": 0.345e-4, "a key": 2}}]
+(1 row)
+

contrib/hstore/hstore--1.1.sql

@@ -234,6 +234,19 @@ LANGUAGE C IMMUTABLE STRICT;
 CREATE CAST (text[] AS hstore)
   WITH FUNCTION hstore(text[]);
 
+CREATE FUNCTION hstore_to_json(hstore)
+RETURNS json
+AS 'MODULE_PATHNAME', 'hstore_to_json'
+LANGUAGE C IMMUTABLE STRICT;
+
+CREATE CAST (hstore AS json)
+  WITH FUNCTION hstore_to_json(hstore);
+
+CREATE FUNCTION hstore_to_json_loose(hstore)
+RETURNS json
+AS 'MODULE_PATHNAME', 'hstore_to_json_loose'
+LANGUAGE C IMMUTABLE STRICT;
+
 CREATE FUNCTION hstore(record)
 RETURNS hstore
 AS 'MODULE_PATHNAME', 'hstore_from_record'

contrib/hstore/hstore_io.c

@@ -8,7 +8,10 @@
 #include "access/htup_details.h"
 #include "catalog/pg_type.h"
 #include "funcapi.h"
+#include "lib/stringinfo.h"
 #include "libpq/pqformat.h"
+#include "utils/builtins.h"
+#include "utils/json.h"
 #include "utils/lsyscache.h"
 #include "utils/typcache.h"
 
@@ -1209,3 +1212,217 @@ hstore_send(PG_FUNCTION_ARGS)
 
 	PG_RETURN_BYTEA_P(pq_endtypsend(&buf));
 }
+
+
+/*
+ * hstore_to_json_loose
+ *
+ * This is a heuristic conversion to json which treats
+ * 't' and 'f' as booleans and strings that look like numbers as numbers,
+ * as long as they don't start with a leading zero followed by another digit
+ * (think zip codes or phone numbers starting with 0).
+ */
+PG_FUNCTION_INFO_V1(hstore_to_json_loose);
+Datum		hstore_to_json_loose(PG_FUNCTION_ARGS);
+Datum
+hstore_to_json_loose(PG_FUNCTION_ARGS)
+{
+	HStore	   *in = PG_GETARG_HS(0);
+	int			buflen,
+				i;
+	int			count = HS_COUNT(in);
+	char	   *out,
+			   *ptr;
+	char	   *base = STRPTR(in);
+	HEntry	   *entries = ARRPTR(in);
+	bool		is_number;
+	StringInfo	src,
+				dst;
+
+	if (count == 0)
+	{
+		out = palloc(1);
+		*out = '\0';
+		PG_RETURN_TEXT_P(cstring_to_text(out));
+	}
+
+	buflen = 3;
+
+	/*
+	 * Formula adjusted slightly from the logic in hstore_out. We have to take
+	 * account of out treatment of booleans to be a bit more pessimistic about
+	 * the length of values.
+	 */
+
+	for (i = 0; i < count; i++)
+	{
+		/* include "" and colon-space and comma-space */
+		buflen += 6 + 2 * HS_KEYLEN(entries, i);
+		/* include "" only if nonnull */
+		buflen += 3 + (HS_VALISNULL(entries, i)
+					   ? 1
+					   : 2 * HS_VALLEN(entries, i));
+	}
+
+	out = ptr = palloc(buflen);
+
+	src = makeStringInfo();
+	dst = makeStringInfo();
+
+	*ptr++ = '{';
+
+	for (i = 0; i < count; i++)
+	{
+		resetStringInfo(src);
+		resetStringInfo(dst);
+		appendBinaryStringInfo(src, HS_KEY(entries, base, i), HS_KEYLEN(entries, i));
+		escape_json(dst, src->data);
+		strncpy(ptr, dst->data, dst->len);
+		ptr += dst->len;
+		*ptr++ = ':';
+		*ptr++ = ' ';
+		resetStringInfo(dst);
+		if (HS_VALISNULL(entries, i))
+			appendStringInfoString(dst, "null");
+		/* guess that values of 't' or 'f' are booleans */
+		else if (HS_VALLEN(entries, i) == 1 && *(HS_VAL(entries, base, i)) == 't')
+			appendStringInfoString(dst, "true");
+		else if (HS_VALLEN(entries, i) == 1 && *(HS_VAL(entries, base, i)) == 'f')
+			appendStringInfoString(dst, "false");
+		else
+		{
+			is_number = false;
+			resetStringInfo(src);
+			appendBinaryStringInfo(src, HS_VAL(entries, base, i), HS_VALLEN(entries, i));
+
+			/*
+			 * don't treat something with a leading zero followed by another
+			 * digit as numeric - could be a zip code or similar
+			 */
+			if (src->len > 0 && (src->data[0] != '0' || !isdigit(src->data[1])) &&
+				strspn(src->data, "+-0123456789Ee.") == src->len)
+			{
+				/*
+				 * might be a number. See if we can input it as a numeric
+				 * value
+				 */
+				char	   *endptr = "junk";
+
+				(void) (strtol(src->data, &endptr, 10) + 1);
+				if (*endptr == '\0')
+				{
+					/*
+					 * strol man page says this means the whole string is
+					 * valid
+					 */
+					is_number = true;
+				}
+				else
+				{
+					/* not an int - try a double */
+					(void) (strtod(src->data, &endptr) + 1.0);
+					if (*endptr == '\0')
+						is_number = true;
+				}
+			}
+			if (is_number)
+				appendBinaryStringInfo(dst, src->data, src->len);
+			else
+				escape_json(dst, src->data);
+		}
+		strncpy(ptr, dst->data, dst->len);
+		ptr += dst->len;
+
+		if (i + 1 != count)
+		{
+			*ptr++ = ',';
+			*ptr++ = ' ';
+		}
+	}
+	*ptr++ = '}';
+	*ptr = '\0';
+
+	PG_RETURN_TEXT_P(cstring_to_text(out));
+}
+
+PG_FUNCTION_INFO_V1(hstore_to_json);
+Datum		hstore_to_json(PG_FUNCTION_ARGS);
+Datum
+hstore_to_json(PG_FUNCTION_ARGS)
+{
+	HStore	   *in = PG_GETARG_HS(0);
+	int			buflen,
+				i;
+	int			count = HS_COUNT(in);
+	char	   *out,
+			   *ptr;
+	char	   *base = STRPTR(in);
+	HEntry	   *entries = ARRPTR(in);
+	StringInfo	src,
+				dst;
+
+	if (count == 0)
+	{
+		out = palloc(1);
+		*out = '\0';
+		PG_RETURN_TEXT_P(cstring_to_text(out));
+	}
+
+	buflen = 3;
+
+	/*
+	 * Formula adjusted slightly from the logic in hstore_out. We have to take
+	 * account of out treatment of booleans to be a bit more pessimistic about
+	 * the length of values.
+	 */
+
+	for (i = 0; i < count; i++)
+	{
+		/* include "" and colon-space and comma-space */
+		buflen += 6 + 2 * HS_KEYLEN(entries, i);
+		/* include "" only if nonnull */
+		buflen += 3 + (HS_VALISNULL(entries, i)
+					   ? 1
+					   : 2 * HS_VALLEN(entries, i));
+	}
+
+	out = ptr = palloc(buflen);
+
+	src = makeStringInfo();
+	dst = makeStringInfo();
+
+	*ptr++ = '{';
+
+	for (i = 0; i < count; i++)
+	{
+		resetStringInfo(src);
+		resetStringInfo(dst);
+		appendBinaryStringInfo(src, HS_KEY(entries, base, i), HS_KEYLEN(entries, i));
+		escape_json(dst, src->data);
+		strncpy(ptr, dst->data, dst->len);
+		ptr += dst->len;
+		*ptr++ = ':';
+		*ptr++ = ' ';
+		resetStringInfo(dst);
+		if (HS_VALISNULL(entries, i))
+			appendStringInfoString(dst, "null");
+		else
+		{
+			resetStringInfo(src);
+			appendBinaryStringInfo(src, HS_VAL(entries, base, i), HS_VALLEN(entries, i));
+			escape_json(dst, src->data);
+		}
+		strncpy(ptr, dst->data, dst->len);
+		ptr += dst->len;
+
+		if (i + 1 != count)
+		{
+			*ptr++ = ',';
+			*ptr++ = ' ';
+		}
+	}
+	*ptr++ = '}';
+	*ptr = '\0';
+
+	PG_RETURN_TEXT_P(cstring_to_text(out));
+}

contrib/hstore/sql/hstore.sql

@@ -330,3 +330,15 @@ set enable_seqscan=off;
 
 select count(*) from testhstore where h #># 'p=>1';
 select count(*) from testhstore where h = 'pos=>98, line=>371, node=>CBA, indexed=>t';
+
+-- json
+select hstore_to_json('"a key" =>1, b => t, c => null, d=> 12345, e => 012345, f=> 1.234, g=> 2.345e+4');
+select cast( hstore  '"a key" =>1, b => t, c => null, d=> 12345, e => 012345, f=> 1.234, g=> 2.345e+4' as json);
+select hstore_to_json_loose('"a key" =>1, b => t, c => null, d=> 12345, e => 012345, f=> 1.234, g=> 2.345e+4');
+
+create table test_json_agg (f1 text, f2 hstore);
+insert into test_json_agg values ('rec1','"a key" =>1, b => t, c => null, d=> 12345, e => 012345, f=> 1.234, g=> 2.345e+4'),
+       ('rec2','"a key" =>2, b => f, c => "null", d=> -12345, e => 012345.6, f=> -1.234, g=> 0.345e-4');
+select json_agg(q) from test_json_agg q;
+select json_agg(q) from (select f1, hstore_to_json_loose(f2) as f2 from test_json_agg) q;
+

doc/src/sgml/func.sgml

@@ -9685,10 +9685,41 @@ table2-mapping
        <entry><literal>row_to_json(row(1,'foo'))</literal></entry>
        <entry><literal>{"f1":1,"f2":"foo"}</literal></entry>
       </row>
+      <row>
+       <entry>
+         <indexterm>
+          <primary>to_json</primary>
+         </indexterm>
+         <literal>to_json(anyelement)</literal>
+       </entry>
+       <entry>
+         Returns the value as JSON. If the data type is not builtin, and there
+         is a cast from the type to json, the cast function will be used to
+         perform the conversion. Otherwise, for any value other than a number,
+         a boolean or NULL, the text representation will be used, escaped and
+         quoted so that it is legal JSON.
+       </entry>
+       <entry><literal>to_json('Fred said "Hi."'</literal></entry>
+       <entry><literal>"Fred said \"Hi.\""</literal></entry>
+      </row>
      </tbody>
     </tgroup>
    </table>
 
+  <note>
+    <para>
+      The <xref linkend="hstore"> extension has a cast from hstore to
+      json, so that converted hstore values are represented as json objects,
+      not as string values.
+    </para>
+  </note>
+
+  <para>
+    See also <xref linkend="functions-aggregate"> about the aggregate
+    function <function>json_agg</function> which aggregates record
+    values as json efficiently.
+  </para>
+
  </sect1>
 
  <sect1 id="functions-sequence">
@@ -11059,6 +11090,22 @@ SELECT NULLIF(value, '(none)') ...
       <entry>equivalent to <function>bool_and</function></entry>
      </row>
 
+     <row>
+      <entry>
+       <indexterm>
+        <primary>json_agg</primary>
+       </indexterm>
+       <function>json_agg(<replaceable class="parameter">record</replaceable>)</function>
+      </entry>
+      <entry>
+       <type>record</type>
+      </entry>
+      <entry>
+       <type>json</type>
+      </entry>
+      <entry>aggregates records as a json array of objects</entry>
+     </row>
+
      <row>
       <entry>
        <indexterm>
@@ -11204,6 +11251,7 @@ SELECT count(*) FROM sometable;
 
   <para>
    The aggregate functions <function>array_agg</function>,
+   <function>json_agg</function>,
    <function>string_agg</function>,
    and <function>xmlagg</function>, as well as similar user-defined
    aggregate functions, produce meaningfully different result values

doc/src/sgml/hstore.sgml

@@ -322,6 +322,22 @@ b
       <entry><literal>{{a,1},{b,2}}</literal></entry>
      </row>
 
+     <row>
+      <entry><function>hstore_to_json(hstore)</function></entry>
+      <entry><type>json</type></entry>
+      <entry>get <type>hstore</type> as a json value</entry>
+      <entry><literal>hstore_to_json('"a key"=&gt;1, b=&gt;t, c=&gt;null, d=&gt;12345, e=&gt;012345, f=&gt;1.234, g=&gt;2.345e+4')</literal></entry>
+      <entry><literal>{"a key": "1", "b": "t", "c": null, "d": "12345", "e": "012345", "f": "1.234", "g": "2.345e+4"}</literal></entry>
+     </row>
+
+     <row>
+      <entry><function>hstore_to_json_loose(hstore)</function></entry>
+      <entry><type>json</type></entry>
+      <entry>get <type>hstore</type> as a json value, but attempting to distinguish numerical and boolean values so they are unquoted in the json</entry>
+      <entry><literal>hstore_to_json('"a key"=&gt;1, b=&gt;t, c=&gt;null, d=&gt;12345, e=&gt;012345, f=&gt;1.234, g=&gt;2.345e+4')</literal></entry>
+      <entry><literal>{"a key": 1, "b": true, "c": null, "d": 12345, "e": "012345", "f": 1.234, "g": 2.345e+4}</literal></entry>
+     </row>
+
      <row>
       <entry><function>slice(hstore, text[])</function></entry>
       <entry><type>hstore</type></entry>
@@ -396,6 +412,13 @@ b
    </tgroup>
   </table>
 
+  <note>
+   <para>
+     The function <function>hstore_to_json</function> is used when an <type>hstore</type>
+     value is cast to <type>json</type>.
+   </para>
+  </note>
+
   <note>
    <para>
     The function <function>populate_record</function> is actually declared

src/backend/utils/adt/json.c

@@ -14,6 +14,8 @@
 #include "postgres.h"
 
 #include "access/htup_details.h"
+#include "access/transam.h"
+#include "catalog/pg_cast.h"
 #include "catalog/pg_type.h"
 #include "executor/spi.h"
 #include "lib/stringinfo.h"
@@ -24,6 +26,7 @@
 #include "utils/builtins.h"
 #include "utils/lsyscache.h"
 #include "utils/json.h"
+#include "utils/syscache.h"
 #include "utils/typcache.h"
 
 typedef enum					/* types of JSON values */
@@ -42,7 +45,7 @@ typedef struct					/* state of JSON lexer */
 {
 	char	   *input;			/* whole string being parsed */
 	char	   *token_start;	/* start of current token within input */
-	char	   *token_terminator; /* end of previous or current token */
+	char	   *token_terminator;		/* end of previous or current token */
 	JsonValueType token_type;	/* type of current token, once it's known */
 } JsonLexContext;
 
@@ -67,7 +70,7 @@ typedef enum					/* required operations on state stack */
 {
 	JSON_STACKOP_NONE,			/* no-op */
 	JSON_STACKOP_PUSH,			/* push new JSON_PARSE_VALUE stack item */
-	JSON_STACKOP_PUSH_WITH_PUSHBACK, /* push, then rescan current token */
+	JSON_STACKOP_PUSH_WITH_PUSHBACK,	/* push, then rescan current token */
 	JSON_STACKOP_POP			/* pop, or expect end of input if no stack */
 } JsonStackOp;
 
@@ -77,19 +80,25 @@ static void json_lex_string(JsonLexContext *lex);
 static void json_lex_number(JsonLexContext *lex, char *s);
 static void report_parse_error(JsonParseStack *stack, JsonLexContext *lex);
 static void report_invalid_token(JsonLexContext *lex);
-static int report_json_context(JsonLexContext *lex);
+static int	report_json_context(JsonLexContext *lex);
 static char *extract_mb_char(char *s);
 static void composite_to_json(Datum composite, StringInfo result,
-							  bool use_line_feeds);
+				  bool use_line_feeds);
 static void array_dim_to_json(StringInfo result, int dim, int ndims, int *dims,
 				  Datum *vals, bool *nulls, int *valcount,
 				  TYPCATEGORY tcategory, Oid typoutputfunc,
 				  bool use_line_feeds);
 static void array_to_json_internal(Datum array, StringInfo result,
-								   bool use_line_feeds);
+					   bool use_line_feeds);
 
+/*
+ * All the defined	type categories are upper case , so use lower case here
+ * so we avoid any possible clash.
+ */
 /* fake type category for JSON so we can distinguish it in datum_to_json */
 #define TYPCATEGORY_JSON 'j'
+/* fake category for types that have a cast to json */
+#define TYPCATEGORY_JSON_CAST 'c'
 /* letters appearing in numeric output that aren't valid in a JSON number */
 #define NON_NUMERIC_LETTER "NnAaIiFfTtYy"
 /* chars to consider as part of an alphanumeric token */
@@ -384,15 +393,15 @@ json_lex(JsonLexContext *lex)
 		 * unintuitive prefix thereof.
 		 */
 		for (p = s; JSON_ALPHANUMERIC_CHAR(*p); p++)
-			/* skip */ ;
+			 /* skip */ ;
 
 		if (p == s)
 		{
 			/*
 			 * We got some sort of unexpected punctuation or an otherwise
 			 * unexpected character, so just complain about that one
-			 * character.  (It can't be multibyte because the above loop
-			 * will advance over any multibyte characters.)
+			 * character.  (It can't be multibyte because the above loop will
+			 * advance over any multibyte characters.)
 			 */
 			lex->token_terminator = s + 1;
 			report_invalid_token(lex);
@@ -585,7 +594,7 @@ json_lex_number(JsonLexContext *lex, char *s)
 	}
 
 	/*
-	 * Check for trailing garbage.  As in json_lex(), any alphanumeric stuff
+	 * Check for trailing garbage.	As in json_lex(), any alphanumeric stuff
 	 * here should be considered part of the token for error-reporting
 	 * purposes.
 	 */
@@ -653,16 +662,16 @@ report_parse_error(JsonParseStack *stack, JsonLexContext *lex)
 				ereport(ERROR,
 						(errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
 						 errmsg("invalid input syntax for type json"),
-						 errdetail("Expected \",\" or \"]\", but found \"%s\".",
-								   token),
+					  errdetail("Expected \",\" or \"]\", but found \"%s\".",
+								token),
 						 report_json_context(lex)));
 				break;
 			case JSON_PARSE_OBJECT_START:
 				ereport(ERROR,
 						(errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
 						 errmsg("invalid input syntax for type json"),
-						 errdetail("Expected string or \"}\", but found \"%s\".",
-								   token),
+					 errdetail("Expected string or \"}\", but found \"%s\".",
+							   token),
 						 report_json_context(lex)));
 				break;
 			case JSON_PARSE_OBJECT_LABEL:
@@ -677,8 +686,8 @@ report_parse_error(JsonParseStack *stack, JsonLexContext *lex)
 				ereport(ERROR,
 						(errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
 						 errmsg("invalid input syntax for type json"),
-						 errdetail("Expected \",\" or \"}\", but found \"%s\".",
-								   token),
+					  errdetail("Expected \",\" or \"}\", but found \"%s\".",
+								token),
 						 report_json_context(lex)));
 				break;
 			case JSON_PARSE_OBJECT_COMMA:
@@ -820,6 +829,7 @@ datum_to_json(Datum val, bool is_null, StringInfo result,
 			  TYPCATEGORY tcategory, Oid typoutputfunc)
 {
 	char	   *outputstr;
+	text	   *jsontext;
 
 	if (is_null)
 	{
@@ -862,6 +872,13 @@ datum_to_json(Datum val, bool is_null, StringInfo result,
 			appendStringInfoString(result, outputstr);
 			pfree(outputstr);
 			break;
+		case TYPCATEGORY_JSON_CAST:
+			jsontext = DatumGetTextP(OidFunctionCall1(typoutputfunc, val));
+			outputstr = text_to_cstring(jsontext);
+			appendStringInfoString(result, outputstr);
+			pfree(outputstr);
+			pfree(jsontext);
+			break;
 		default:
 			outputstr = OidOutputFunctionCall(typoutputfunc, val);
 			escape_json(result, outputstr);
@@ -935,6 +952,7 @@ array_to_json_internal(Datum array, StringInfo result, bool use_line_feeds)
 	Oid			typioparam;
 	Oid			typoutputfunc;
 	TYPCATEGORY tcategory;
+	Oid			castfunc = InvalidOid;
 
 	ndim = ARR_NDIM(v);
 	dim = ARR_DIMS(v);
@@ -950,11 +968,32 @@ array_to_json_internal(Datum array, StringInfo result, bool use_line_feeds)
 					 &typlen, &typbyval, &typalign,
 					 &typdelim, &typioparam, &typoutputfunc);
 
+	if (element_type > FirstNormalObjectId)
+	{
+		HeapTuple	tuple;
+		Form_pg_cast castForm;
+
+		tuple = SearchSysCache2(CASTSOURCETARGET,
+								ObjectIdGetDatum(element_type),
+								ObjectIdGetDatum(JSONOID));
+		if (HeapTupleIsValid(tuple))
+		{
+			castForm = (Form_pg_cast) GETSTRUCT(tuple);
+
+			if (castForm->castmethod == COERCION_METHOD_FUNCTION)
+				castfunc = typoutputfunc = castForm->castfunc;
+
+			ReleaseSysCache(tuple);
+		}
+	}
+
 	deconstruct_array(v, element_type, typlen, typbyval,
 					  typalign, &elements, &nulls,
 					  &nitems);
 
-	if (element_type == RECORDOID)
+	if (castfunc != InvalidOid)
+		tcategory = TYPCATEGORY_JSON_CAST;
+	else if (element_type == RECORDOID)
 		tcategory = TYPCATEGORY_COMPOSITE;
 	else if (element_type == JSONOID)
 		tcategory = TYPCATEGORY_JSON;
@@ -1009,6 +1048,7 @@ composite_to_json(Datum composite, StringInfo result, bool use_line_feeds)
 		TYPCATEGORY tcategory;
 		Oid			typoutput;
 		bool		typisvarlena;
+		Oid			castfunc = InvalidOid;
 
 		if (tupdesc->attrs[i]->attisdropped)
 			continue;
@@ -1023,7 +1063,31 @@ composite_to_json(Datum composite, StringInfo result, bool use_line_feeds)
 
 		origval = heap_getattr(tuple, i + 1, tupdesc, &isnull);
 
-		if (tupdesc->attrs[i]->atttypid == RECORDARRAYOID)
+		getTypeOutputInfo(tupdesc->attrs[i]->atttypid,
+						  &typoutput, &typisvarlena);
+
+		if (tupdesc->attrs[i]->atttypid > FirstNormalObjectId)
+		{
+			HeapTuple	cast_tuple;
+			Form_pg_cast castForm;
+
+			cast_tuple = SearchSysCache2(CASTSOURCETARGET,
+							   ObjectIdGetDatum(tupdesc->attrs[i]->atttypid),
+										 ObjectIdGetDatum(JSONOID));
+			if (HeapTupleIsValid(cast_tuple))
+			{
+				castForm = (Form_pg_cast) GETSTRUCT(cast_tuple);
+
+				if (castForm->castmethod == COERCION_METHOD_FUNCTION)
+					castfunc = typoutput = castForm->castfunc;
+
+				ReleaseSysCache(cast_tuple);
+			}
+		}
+
+		if (castfunc != InvalidOid)
+			tcategory = TYPCATEGORY_JSON_CAST;
+		else if (tupdesc->attrs[i]->atttypid == RECORDARRAYOID)
 			tcategory = TYPCATEGORY_ARRAY;
 		else if (tupdesc->attrs[i]->atttypid == RECORDOID)
 			tcategory = TYPCATEGORY_COMPOSITE;
@@ -1032,9 +1096,6 @@ composite_to_json(Datum composite, StringInfo result, bool use_line_feeds)
 		else
 			tcategory = TypeCategory(tupdesc->attrs[i]->atttypid);
 
-		getTypeOutputInfo(tupdesc->attrs[i]->atttypid,
-						  &typoutput, &typisvarlena);
-
 		/*
 		 * If we have a toasted datum, forcibly detoast it here to avoid
 		 * memory leakage inside the type's output routine.
@@ -1121,6 +1182,222 @@ row_to_json_pretty(PG_FUNCTION_ARGS)
 	PG_RETURN_TEXT_P(cstring_to_text(result->data));
 }
 
+/*
+ * SQL function to_json(anyvalue)
+ */
+Datum
+to_json(PG_FUNCTION_ARGS)
+{
+	Oid			val_type = get_fn_expr_argtype(fcinfo->flinfo, 0);
+	StringInfo	result;
+	Datum		orig_val,
+				val;
+	TYPCATEGORY tcategory;
+	Oid			typoutput;
+	bool		typisvarlena;
+	Oid			castfunc = InvalidOid;
+
+	if (val_type == InvalidOid)
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				 errmsg("could not determine input data type")));
+
+
+	result = makeStringInfo();
+
+	orig_val = PG_ARGISNULL(0) ? (Datum) 0 : PG_GETARG_DATUM(0);
+
+	getTypeOutputInfo(val_type, &typoutput, &typisvarlena);
+
+	if (val_type > FirstNormalObjectId)
+	{
+		HeapTuple	tuple;
+		Form_pg_cast castForm;
+
+		tuple = SearchSysCache2(CASTSOURCETARGET,
+								ObjectIdGetDatum(val_type),
+								ObjectIdGetDatum(JSONOID));
+		if (HeapTupleIsValid(tuple))
+		{
+			castForm = (Form_pg_cast) GETSTRUCT(tuple);
+
+			if (castForm->castmethod == COERCION_METHOD_FUNCTION)
+				castfunc = typoutput = castForm->castfunc;
+
+			ReleaseSysCache(tuple);
+		}
+	}
+
+	if (castfunc != InvalidOid)
+		tcategory = TYPCATEGORY_JSON_CAST;
+	else if (val_type == RECORDARRAYOID)
+		tcategory = TYPCATEGORY_ARRAY;
+	else if (val_type == RECORDOID)
+		tcategory = TYPCATEGORY_COMPOSITE;
+	else if (val_type == JSONOID)
+		tcategory = TYPCATEGORY_JSON;
+	else
+		tcategory = TypeCategory(val_type);
+
+	/*
+	 * If we have a toasted datum, forcibly detoast it here to avoid memory
+	 * leakage inside the type's output routine.
+	 */
+	if (typisvarlena && orig_val != (Datum) 0)
+		val = PointerGetDatum(PG_DETOAST_DATUM(orig_val));
+	else
+		val = orig_val;
+
+	datum_to_json(val, false, result, tcategory, typoutput);
+
+	/* Clean up detoasted copy, if any */
+	if (val != orig_val)
+		pfree(DatumGetPointer(val));
+
+	PG_RETURN_TEXT_P(cstring_to_text(result->data));
+}
+
+/*
+ * json_agg transition function
+ */
+Datum
+json_agg_transfn(PG_FUNCTION_ARGS)
+{
+	Oid			val_type = get_fn_expr_argtype(fcinfo->flinfo, 1);
+	MemoryContext aggcontext,
+				oldcontext;
+	StringInfo	state;
+	Datum		orig_val,
+				val;
+	TYPCATEGORY tcategory;
+	Oid			typoutput;
+	bool		typisvarlena;
+	Oid			castfunc = InvalidOid;
+
+	if (val_type == InvalidOid)
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				 errmsg("could not determine input data type")));
+
+	if (!AggCheckCallContext(fcinfo, &aggcontext))
+	{
+		/* cannot be called directly because of internal-type argument */
+		elog(ERROR, "json_agg_transfn called in non-aggregate context");
+	}
+
+	if (PG_ARGISNULL(0))
+	{
+		/*
+		 * Make this StringInfo in a context where it will persist for the
+		 * duration off the aggregate call. It's only needed for this initial
+		 * piece, as the StringInfo routines make sure they use the right
+		 * context to enlarge the object if necessary.
+		 */
+		oldcontext = MemoryContextSwitchTo(aggcontext);
+		state = makeStringInfo();
+		MemoryContextSwitchTo(oldcontext);
+
+		appendStringInfoChar(state, '[');
+	}
+	else
+	{
+		state = (StringInfo) PG_GETARG_POINTER(0);
+		appendStringInfoString(state, ", ");
+	}
+
+	/* fast path for NULLs */
+	if (PG_ARGISNULL(1))
+	{
+		orig_val = (Datum) 0;
+		datum_to_json(orig_val, true, state, 0, InvalidOid);
+		PG_RETURN_POINTER(state);
+	}
+
+
+	orig_val = PG_GETARG_DATUM(1);
+
+	getTypeOutputInfo(val_type, &typoutput, &typisvarlena);
+
+	if (val_type > FirstNormalObjectId)
+	{
+		HeapTuple	tuple;
+		Form_pg_cast castForm;
+
+		tuple = SearchSysCache2(CASTSOURCETARGET,
+								ObjectIdGetDatum(val_type),
+								ObjectIdGetDatum(JSONOID));
+		if (HeapTupleIsValid(tuple))
+		{
+			castForm = (Form_pg_cast) GETSTRUCT(tuple);
+
+			if (castForm->castmethod == COERCION_METHOD_FUNCTION)
+				castfunc = typoutput = castForm->castfunc;
+
+			ReleaseSysCache(tuple);
+		}
+	}
+
+	if (castfunc != InvalidOid)
+		tcategory = TYPCATEGORY_JSON_CAST;
+	else if (val_type == RECORDARRAYOID)
+		tcategory = TYPCATEGORY_ARRAY;
+	else if (val_type == RECORDOID)
+		tcategory = TYPCATEGORY_COMPOSITE;
+	else if (val_type == JSONOID)
+		tcategory = TYPCATEGORY_JSON;
+	else
+		tcategory = TypeCategory(val_type);
+
+	/*
+	 * If we have a toasted datum, forcibly detoast it here to avoid memory
+	 * leakage inside the type's output routine.
+	 */
+	if (typisvarlena)
+		val = PointerGetDatum(PG_DETOAST_DATUM(orig_val));
+	else
+		val = orig_val;
+
+	if (!PG_ARGISNULL(0) &&
+	  (tcategory == TYPCATEGORY_ARRAY || tcategory == TYPCATEGORY_COMPOSITE))
+	{
+		appendStringInfoString(state, "\n ");
+	}
+
+	datum_to_json(val, false, state, tcategory, typoutput);
+
+	/* Clean up detoasted copy, if any */
+	if (val != orig_val)
+		pfree(DatumGetPointer(val));
+
+	/*
+	 * The transition type for array_agg() is declared to be "internal", which
+	 * is a pass-by-value type the same size as a pointer.	So we can safely
+	 * pass the ArrayBuildState pointer through nodeAgg.c's machinations.
+	 */
+	PG_RETURN_POINTER(state);
+}
+
+/*
+ * json_agg final function
+ */
+Datum
+json_agg_finalfn(PG_FUNCTION_ARGS)
+{
+	StringInfo	state;
+
+	/* cannot be called directly because of internal-type argument */
+	Assert(AggCheckCallContext(fcinfo, NULL));
+
+	state = PG_ARGISNULL(0) ? NULL : (StringInfo) PG_GETARG_POINTER(0);
+
+	if (state == NULL)
+		PG_RETURN_NULL();
+
+	appendStringInfoChar(state, ']');
+
+	PG_RETURN_TEXT_P(cstring_to_text(state->data));
+}
+
 /*
  * Produce a JSON string literal, properly escaping characters in the text.
  */

src/include/catalog/catversion.h

@@ -53,6 +53,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	201303081
+#define CATALOG_VERSION_NO	201303101
 
 #endif

src/include/catalog/pg_aggregate.h

@@ -232,6 +232,9 @@ DATA(insert ( 3538	string_agg_transfn	string_agg_finalfn		0	2281	_null_ ));
 /* bytea */
 DATA(insert ( 3545	bytea_string_agg_transfn	bytea_string_agg_finalfn		0	2281	_null_ ));
 
+/* json */
+DATA(insert ( 3175	json_agg_transfn	json_agg_finalfn		0	2281	_null_ ));
+
 /*
  * prototypes for functions in pg_aggregate.c
  */

src/include/catalog/pg_proc.h

@@ -4106,6 +4106,14 @@ DATA(insert OID = 3155 (  row_to_json	   PGNSP PGUID 12 1 0 0 0 f f f f t f s 1
 DESCR("map row to json");
 DATA(insert OID = 3156 (  row_to_json	   PGNSP PGUID 12 1 0 0 0 f f f f t f s 2 0 114 "2249 16" _null_ _null_ _null_ _null_ row_to_json_pretty _null_ _null_ _null_ ));
 DESCR("map row to json with optional pretty printing");
+DATA(insert OID = 3173 (  json_agg_transfn   PGNSP PGUID 12 1 0 0 0 f f f f f f i 2 0 2281 "2281 2283" _null_ _null_ _null_ _null_ json_agg_transfn _null_ _null_ _null_ ));
+DESCR("json aggregate transition function");
+DATA(insert OID = 3174 (  json_agg_finalfn   PGNSP PGUID 12 1 0 0 0 f f f f f f i 1 0 114 "2281" _null_ _null_ _null_ _null_ json_agg_finalfn _null_ _null_ _null_ ));
+DESCR("json aggregate final function");
+DATA(insert OID = 3175 (  json_agg		   PGNSP PGUID 12 1 0 0 0 t f f f f f i 1 0 114 "2283" _null_ _null_ _null_ _null_ aggregate_dummy _null_ _null_ _null_ ));
+DESCR("aggregate input into json");
+DATA(insert OID = 3176 (  to_json	   PGNSP PGUID 12 1 0 0 0 f f f f t f s 1 0 114 "2283" _null_ _null_ _null_ _null_ to_json _null_ _null_ _null_ ));
+DESCR("map input to json");
 
 /* uuid */
 DATA(insert OID = 2952 (  uuid_in		   PGNSP PGUID 12 1 0 0 0 f f f f t f i 1 0 2950 "2275" _null_ _null_ _null_ _null_ uuid_in _null_ _null_ _null_ ));

src/include/utils/json.h

@@ -25,6 +25,11 @@ extern Datum array_to_json(PG_FUNCTION_ARGS);
 extern Datum array_to_json_pretty(PG_FUNCTION_ARGS);
 extern Datum row_to_json(PG_FUNCTION_ARGS);
 extern Datum row_to_json_pretty(PG_FUNCTION_ARGS);
+extern Datum to_json(PG_FUNCTION_ARGS);
+
+extern Datum json_agg_transfn(PG_FUNCTION_ARGS);
+extern Datum json_agg_finalfn(PG_FUNCTION_ARGS);
+
 extern void escape_json(StringInfo buf, const char *str);
 
 #endif   /* JSON_H */

src/test/regress/expected/json.out

@@ -403,6 +403,30 @@ SELECT row_to_json(row((select array_agg(x) as d from generate_series(5,10) x)),
  {"f1":[5,6,7,8,9,10]}
 (1 row)
 
+--json_agg
+SELECT json_agg(q)
+  FROM ( SELECT $$a$$ || x AS b, y AS c,
+               ARRAY[ROW(x.*,ARRAY[1,2,3]),
+               ROW(y.*,ARRAY[4,5,6])] AS z
+         FROM generate_series(1,2) x,
+              generate_series(4,5) y) q;
+                               json_agg                                
+-----------------------------------------------------------------------
+ [{"b":"a1","c":4,"z":[{"f1":1,"f2":[1,2,3]},{"f1":4,"f2":[4,5,6]}]}, +
+  {"b":"a1","c":5,"z":[{"f1":1,"f2":[1,2,3]},{"f1":5,"f2":[4,5,6]}]}, +
+  {"b":"a2","c":4,"z":[{"f1":2,"f2":[1,2,3]},{"f1":4,"f2":[4,5,6]}]}, +
+  {"b":"a2","c":5,"z":[{"f1":2,"f2":[1,2,3]},{"f1":5,"f2":[4,5,6]}]}]
+(1 row)
+
+SELECT json_agg(q)
+  FROM rows q;
+       json_agg        
+-----------------------
+ [{"x":1,"y":"txt1"}, +
+  {"x":2,"y":"txt2"}, +
+  {"x":3,"y":"txt3"}]
+(1 row)
+
 -- non-numeric output
 SELECT row_to_json(q)
 FROM (SELECT 'NaN'::float8 AS "float8field") q;

src/test/regress/sql/json.sql

@@ -100,6 +100,18 @@ FROM rows q;
 
 SELECT row_to_json(row((select array_agg(x) as d from generate_series(5,10) x)),false);
 
+--json_agg
+
+SELECT json_agg(q)
+  FROM ( SELECT $$a$$ || x AS b, y AS c,
+               ARRAY[ROW(x.*,ARRAY[1,2,3]),
+               ROW(y.*,ARRAY[4,5,6])] AS z
+         FROM generate_series(1,2) x,
+              generate_series(4,5) y) q;
+
+SELECT json_agg(q)
+  FROM rows q;
+
 -- non-numeric output
 SELECT row_to_json(q)
 FROM (SELECT 'NaN'::float8 AS "float8field") q;