Correct volatility markings of a few json functions.

json_agg and json_object_agg and their associated transition functions
should have been marked as stable rather than immutable, as they call IO
functions indirectly. Changing this probably isn't going to make much
difference, as you can't use an aggregate function in an index
expression, but we should be correct nevertheless.

json_object, on the other hand, should be marked immutable rather than
stable, as it does not call IO functions.

As discussed on -hackers, this change is being made without bumping the
catalog version, as we don't want to do that at this stage of the  cycle,
and  the changes are very unlikely to affect anyone.

Author: adunstan

src/include/catalog/pg_proc.h

@@ -4197,17 +4197,17 @@ 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_ ));
+DATA(insert OID = 3173 (  json_agg_transfn	 PGNSP PGUID 12 1 0 0 0 f f f f f f s 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_ ));
+DATA(insert OID = 3175 (  json_agg		   PGNSP PGUID 12 1 0 0 0 t f f f f f s 1 0 114 "2283" _null_ _null_ _null_ _null_ aggregate_dummy _null_ _null_ _null_ ));
 DESCR("aggregate input into json");
-DATA(insert OID = 3180 (  json_object_agg_transfn	 PGNSP PGUID 12 1 0 0 0 f f f f f f i 3 0 2281 "2281 2276 2276" _null_ _null_ _null_ _null_ json_object_agg_transfn _null_ _null_ _null_ ));
+DATA(insert OID = 3180 (  json_object_agg_transfn	 PGNSP PGUID 12 1 0 0 0 f f f f f f s 3 0 2281 "2281 2276 2276" _null_ _null_ _null_ _null_ json_object_agg_transfn _null_ _null_ _null_ ));
 DESCR("json object aggregate transition function");
 DATA(insert OID = 3196 (  json_object_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_object_agg_finalfn _null_ _null_ _null_ ));
 DESCR("json object aggregate final function");
-DATA(insert OID = 3197 (  json_object_agg		   PGNSP PGUID 12 1 0 0 0 t f f f f f i 2 0 114 "2276 2276" _null_ _null_ _null_ _null_ aggregate_dummy _null_ _null_ _null_ ));
+DATA(insert OID = 3197 (  json_object_agg		   PGNSP PGUID 12 1 0 0 0 t f f f f f s 2 0 114 "2276 2276" _null_ _null_ _null_ _null_ aggregate_dummy _null_ _null_ _null_ ));
 DESCR("aggregate input into a json object");
 DATA(insert OID = 3198 (  json_build_array	   PGNSP PGUID 12 1 0 2276 0 f f f f f f s 1 0 114 "2276" "{2276}" "{v}" _null_ _null_ json_build_array _null_ _null_ _null_ ));
 DESCR("build a json array from any inputs");
@@ -4217,9 +4217,9 @@ DATA(insert OID = 3200 (  json_build_object    PGNSP PGUID 12 1 0 2276 0 f f f f
 DESCR("build a json object from pairwise key/value inputs");
 DATA(insert OID = 3201 (  json_build_object    PGNSP PGUID 12 1 0 0 0 f f f f f f s 0 0 114  "" _null_ _null_ _null_ _null_ json_build_object_noargs _null_ _null_ _null_ ));
 DESCR("build an empty json object");
-DATA(insert OID = 3202 (  json_object	 PGNSP PGUID 12 1 0 0 0 f f f f t f s 1 0 114 "1009" _null_ _null_ _null_ _null_ json_object _null_ _null_ _null_ ));
+DATA(insert OID = 3202 (  json_object	 PGNSP PGUID 12 1 0 0 0 f f f f t f i 1 0 114 "1009" _null_ _null_ _null_ _null_ json_object _null_ _null_ _null_ ));
 DESCR("map text array of key value pairs to json object");
-DATA(insert OID = 3203 (  json_object	 PGNSP PGUID 12 1 0 0 0 f f f f t f s 2 0 114 "1009 1009" _null_ _null_ _null_ _null_ json_object_two_arg _null_ _null_ _null_ ));
+DATA(insert OID = 3203 (  json_object	 PGNSP PGUID 12 1 0 0 0 f f f f t f i 2 0 114 "1009 1009" _null_ _null_ _null_ _null_ json_object_two_arg _null_ _null_ _null_ ));
 DESCR("map text arrays of keys and values to json object");
 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");