SQL/JSON query functions

This introduces the SQL/JSON functions for querying JSON data using
jsonpath expressions. The functions are:

JSON_EXISTS()
JSON_QUERY()
JSON_VALUE()

All of these functions only operate on jsonb. The workaround for now is
to cast the argument to jsonb.

JSON_EXISTS() tests if the jsonpath expression applied to the jsonb
value yields any values. JSON_VALUE() must return a single value, and an
error occurs if it tries to return multiple values. JSON_QUERY() must
return a json object or array, and there are various WRAPPER options for
handling scalar or multi-value results. Both these functions have
options for handling EMPTY and ERROR conditions.

Nikita Glukhov

Reviewers have included (in no particular order) Andres Freund, Alexander
Korotkov, Pavel Stehule, Andrew Alsup, Erik Rijkers, Zihong Yu,
Himanshu Upadhyaya, Daniel Gustafsson, Justin Pryzby.

Discussion: https://postgr.es/m/cd0bb935-0158-78a7-08b5-904886deac4b@postgrespro.ru

1 files changed, 62 insertions, 0 deletions

diff --git a/src/backend/utils/adt/jsonb.c b/src/backend/utils/adt/jsonb.c
index a103cbc7c69..d383cbdfedb 100644
--- a/src/backend/utils/adt/jsonb.c
+++ b/src/backend/utils/adt/jsonb.c
@@ -2227,3 +2227,65 @@ jsonb_float8(PG_FUNCTION_ARGS)
 
 	PG_RETURN_DATUM(retValue);
 }
+
+/*
+ * Construct an empty array jsonb.
+ */
+Jsonb *
+JsonbMakeEmptyArray(void)
+{
+	JsonbValue jbv;
+
+	jbv.type = jbvArray;
+	jbv.val.array.elems = NULL;
+	jbv.val.array.nElems = 0;
+	jbv.val.array.rawScalar = false;
+
+	return JsonbValueToJsonb(&jbv);
+}
+
+/*
+ * Construct an empty object jsonb.
+ */
+Jsonb *
+JsonbMakeEmptyObject(void)
+{
+	JsonbValue jbv;
+
+	jbv.type = jbvObject;
+	jbv.val.object.pairs = NULL;
+	jbv.val.object.nPairs = 0;
+
+	return JsonbValueToJsonb(&jbv);
+}
+
+/*
+ * Convert jsonb to a C-string stripping quotes from scalar strings.
+ */
+char *
+JsonbUnquote(Jsonb *jb)
+{
+	if (JB_ROOT_IS_SCALAR(jb))
+	{
+		JsonbValue	v;
+
+		JsonbExtractScalar(&jb->root, &v);
+
+		if (v.type == jbvString)
+			return pnstrdup(v.val.string.val, v.val.string.len);
+		else if (v.type == jbvBool)
+			return pstrdup(v.val.boolean ? "true" : "false");
+		else if (v.type == jbvNumeric)
+			return DatumGetCString(DirectFunctionCall1(numeric_out,
+									   PointerGetDatum(v.val.numeric)));
+		else if (v.type == jbvNull)
+			return pstrdup("null");
+		else
+		{
+			elog(ERROR, "unrecognized jsonb value type %d", v.type);
+			return NULL;
+		}
+	}
+	else
+		return JsonbToCString(NULL, &jb->root, VARSIZE(jb));
+}