SQL/JSON: add standard JSON constructor functions

This commit introduces the SQL/JSON standard-conforming constructors for
JSON types:

JSON_ARRAY()
JSON_ARRAYAGG()
JSON_OBJECT()
JSON_OBJECTAGG()

Most of the functionality was already present in PostgreSQL-specific
functions, but these include some new functionality such as the ability
to skip or include NULL values, and to allow duplicate keys or throw
error when they are found, as well as the standard specified syntax to
specify output type and format.

Author: Nikita Glukhov <n.gluhov@postgrespro.ru>
Author: Teodor Sigaev <teodor@sigaev.ru>
Author: Oleg Bartunov <obartunov@gmail.com>
Author: Alexander Korotkov <aekorotkov@gmail.com>
Author: Amit Langote <amitlangote09@gmail.com>

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/CAF4Au4w2x-5LTnN_bxky-mq4=WOqsGsxSpENCzHRAzSnEd8+WQ@mail.gmail.com
Discussion: https://postgr.es/m/cd0bb935-0158-78a7-08b5-904886deac4b@postgrespro.ru
Discussion: https://postgr.es/m/20220616233130.rparivafipt6doj3@alap3.anarazel.de
Discussion: https://postgr.es/m/abd9b83b-aa66-f230-3d6d-734817f0995d%40postgresql.org

2 files changed, 141 insertions, 0 deletions

diff --git a/src/backend/executor/execExpr.c b/src/backend/executor/execExpr.c
index c61f23c6c18..6c5a3780292 100644
--- a/src/backend/executor/execExpr.c
+++ b/src/backend/executor/execExpr.c
@@ -2279,6 +2279,97 @@ ExecInitExprRec(Expr *node, ExprState *state,
 				break;
 			}
 
+		case T_JsonValueExpr:
+			{
+				JsonValueExpr *jve = (JsonValueExpr *) node;
+
+				ExecInitExprRec(jve->raw_expr, state, resv, resnull);
+
+				if (jve->formatted_expr)
+				{
+					Datum	   *innermost_caseval = state->innermost_caseval;
+					bool	   *innermost_isnull = state->innermost_casenull;
+
+					state->innermost_caseval = resv;
+					state->innermost_casenull = resnull;
+
+					ExecInitExprRec(jve->formatted_expr, state, resv, resnull);
+
+					state->innermost_caseval = innermost_caseval;
+					state->innermost_casenull = innermost_isnull;
+				}
+				break;
+			}
+
+		case T_JsonConstructorExpr:
+			{
+				JsonConstructorExpr *ctor = (JsonConstructorExpr *) node;
+				List	   *args = ctor->args;
+				ListCell   *lc;
+				int			nargs = list_length(args);
+				int			argno = 0;
+
+				if (ctor->func)
+				{
+					ExecInitExprRec(ctor->func, state, resv, resnull);
+				}
+				else
+				{
+					JsonConstructorExprState *jcstate;
+
+					jcstate = palloc0(sizeof(JsonConstructorExprState));
+
+					scratch.opcode = EEOP_JSON_CONSTRUCTOR;
+					scratch.d.json_constructor.jcstate = jcstate;
+
+					jcstate->constructor = ctor;
+					jcstate->arg_values = (Datum *) palloc(sizeof(Datum) * nargs);
+					jcstate->arg_nulls = (bool *) palloc(sizeof(bool) * nargs);
+					jcstate->arg_types = (Oid *) palloc(sizeof(Oid) * nargs);
+					jcstate->nargs = nargs;
+
+					foreach(lc, args)
+					{
+						Expr	   *arg = (Expr *) lfirst(lc);
+
+						jcstate->arg_types[argno] = exprType((Node *) arg);
+
+						if (IsA(arg, Const))
+						{
+							/* Don't evaluate const arguments every round */
+							Const	   *con = (Const *) arg;
+
+							jcstate->arg_values[argno] = con->constvalue;
+							jcstate->arg_nulls[argno] = con->constisnull;
+						}
+						else
+						{
+							ExecInitExprRec(arg, state,
+											&jcstate->arg_values[argno],
+											&jcstate->arg_nulls[argno]);
+						}
+						argno++;
+					}
+
+					ExprEvalPushStep(state, &scratch);
+				}
+
+				if (ctor->coercion)
+				{
+					Datum	   *innermost_caseval = state->innermost_caseval;
+					bool	   *innermost_isnull = state->innermost_casenull;
+
+					state->innermost_caseval = resv;
+					state->innermost_casenull = resnull;
+
+					ExecInitExprRec(ctor->coercion, state, resv, resnull);
+
+					state->innermost_caseval = innermost_caseval;
+					state->innermost_casenull = innermost_isnull;
+				}
+			}
+			break;
+
 		case T_NullTest:
 			{
 				NullTest   *ntest = (NullTest *) node;
diff --git a/src/backend/executor/execExprInterp.c b/src/backend/executor/execExprInterp.c
index dd7d1af220a..a37ba4dd55b 100644
--- a/src/backend/executor/execExprInterp.c
+++ b/src/backend/executor/execExprInterp.c
@@ -71,6 +71,8 @@
 #include "utils/date.h"
 #include "utils/datum.h"
 #include "utils/expandedrecord.h"
+#include "utils/json.h"
+#include "utils/jsonb.h"
 #include "utils/lsyscache.h"
 #include "utils/memutils.h"
 #include "utils/timestamp.h"
@@ -474,6 +476,7 @@ ExecInterpExpr(ExprState *state, ExprContext *econtext, bool *isnull)
 		&&CASE_EEOP_SCALARARRAYOP,
 		&&CASE_EEOP_HASHED_SCALARARRAYOP,
 		&&CASE_EEOP_XMLEXPR,
+		&&CASE_EEOP_JSON_CONSTRUCTOR,
 		&&CASE_EEOP_AGGREF,
 		&&CASE_EEOP_GROUPING_FUNC,
 		&&CASE_EEOP_WINDOW_FUNC,
@@ -1511,6 +1514,13 @@ ExecInterpExpr(ExprState *state, ExprContext *econtext, bool *isnull)
 			EEO_NEXT();
 		}
 
+		EEO_CASE(EEOP_JSON_CONSTRUCTOR)
+		{
+			/* too complex for an inline implementation */
+			ExecEvalJsonConstructor(state, op, econtext);
+			EEO_NEXT();
+		}
+
 		EEO_CASE(EEOP_AGGREF)
 		{
 			/*
@@ -4437,3 +4447,43 @@ ExecAggPlainTransByRef(AggState *aggstate, AggStatePerTrans pertrans,
 
 	MemoryContextSwitchTo(oldContext);
 }
+
+/*
+ * Evaluate a JSON constructor expression.
+ */
+void
+ExecEvalJsonConstructor(ExprState *state, ExprEvalStep *op,
+						ExprContext *econtext)
+{
+	Datum		res;
+	JsonConstructorExprState *jcstate = op->d.json_constructor.jcstate;
+	JsonConstructorExpr *ctor = jcstate->constructor;
+	bool		is_jsonb = ctor->returning->format->format_type == JS_FORMAT_JSONB;
+	bool		isnull = false;
+
+	if (ctor->type == JSCTOR_JSON_ARRAY)
+		res = (is_jsonb ?
+			   jsonb_build_array_worker :
+			   json_build_array_worker) (jcstate->nargs,
+										 jcstate->arg_values,
+										 jcstate->arg_nulls,
+										 jcstate->arg_types,
+										 jcstate->constructor->absent_on_null);
+	else if (ctor->type == JSCTOR_JSON_OBJECT)
+		res = (is_jsonb ?
+			   jsonb_build_object_worker :
+			   json_build_object_worker) (jcstate->nargs,
+										  jcstate->arg_values,
+										  jcstate->arg_nulls,
+										  jcstate->arg_types,
+										  jcstate->constructor->absent_on_null,
+										  jcstate->constructor->unique);
+	else
+	{
+		res = (Datum) 0;
+		elog(ERROR, "invalid JsonConstructorExpr type %d", ctor->type);
+	}
+
+	*op->resvalue = res;
+	*op->resnull = isnull;
+}