MVT: Improve performance and memory usage

- Reduce memory usage (raises protobuf-c requirement to 1.1).
- Improves performance when merging tiles (from parallel queries).
- Uses protobuf values to reduce the number of transformations and
  memory usage.
- Uses a temporal context when transforming data (reading rows) to
  have a fast way of cleaning up memory before the parallel process
  starts.
- Processes texts and cstrings with each known reading functions
  instead of the `OidOutputFunctionCall` fallback.
- Avoids multiple hash function calls when a value is not found in
  the hash table (once for the lookup and once for the insertion).

Closes #576
Closes #4737

Author: Algunenano

NEWS

@@ -2,6 +2,9 @@ PostGIS 3.1.0beta1
 2020/xx/xx
 Only tickets not included in 3.1.0alpha2
 
+* Breaking changes *
+  - #4737, Bump minimum protobuf-c requirement to 1.1.0 (Raúl Marín)
+
 * New features *
   - #4698, Add a precision parameter to ST_AsEWKT (Raúl Marín)
 
@@ -23,6 +26,8 @@ Only tickets not included in 3.1.0alpha2
            ST_BoundingDiagonal.
   - #4741, Don't use ST_PointInsideCircle if you need indexes, use ST_DWithin instead.
            Documentation adjusted (Darafei Praliaskouski)
+  - #4737, Improve performance and reduce memory usage in ST_AsMVT, especially in
+           queries involving parallelism (Raúl Marín)
 
 * Bug fixes *
   - #4691, Fix segfault during gist index creation with empty geometries (Raúl Marín)

configure.ac

@@ -998,7 +998,7 @@ if test "$CHECK_PROTOBUF" != "no"; then
 	dnl Try pkgconfig first
 	if test -n "$PKG_CONFIG"; then
 		dnl Ensure libprotobuf-c is of minimum required version
-		PKG_CHECK_MODULES([PROTOBUFC], [libprotobuf-c >= 1.0.0], [
+		PKG_CHECK_MODULES([PROTOBUFC], [libprotobuf-c >= 1.1.0], [
 				PROTOBUF_CPPFLAGS="$PROTOBUFC_CFLAGS";
 				PROTOBUF_LDFLAGS="$PROTOBUFC_LIBS";
 			], [
@@ -1069,10 +1069,8 @@ if test "$CHECK_PROTOBUF" != "no"; then
         if test "$HAVE_PROTOBUF" = "yes"; then
 		AC_DEFINE([HAVE_LIBPROTOBUF], [1], [Define to 1 if libprotobuf-c is present])
 		AC_DEFINE_UNQUOTED([LIBPROTOBUF_VERSION], [$PROTOC_VERSION], [Numeric version number for libprotobuf-c])
-		if test $PROTOC_VERSION -ge 1001000; then
-			AC_DEFINE([HAVE_GEOBUF], [1], [Define to 1 if libprotobuf is >= 1.1])
-			HAVE_GEOBUF=yes
-		fi
+		AC_DEFINE([HAVE_GEOBUF], [1], [Define to 1 if geobuf is present])
+		HAVE_GEOBUF=yes
 	fi
 
     AC_SUBST([HAVE_PROTOBUF])

postgis/lwgeom_out_mvt.c

@@ -129,14 +129,14 @@ Datum pgis_asmvt_transfn(PG_FUNCTION_ARGS)
 	elog(ERROR, "Missing libprotobuf-c");
 	PG_RETURN_NULL();
 #else
-	MemoryContext aggcontext;
+	MemoryContext aggcontext, old_context;
 	mvt_agg_context *ctx;
 
 	if (!AggCheckCallContext(fcinfo, &aggcontext))
 		elog(ERROR, "%s called in non-aggregate context", __func__);
-	MemoryContextSwitchTo(aggcontext);
 
 	if (PG_ARGISNULL(0)) {
+		old_context = MemoryContextSwitchTo(aggcontext);
 		ctx = palloc(sizeof(*ctx));
 		ctx->name = "default";
 		if (PG_NARGS() > 2 && !PG_ARGISNULL(2))
@@ -151,7 +151,12 @@ Datum pgis_asmvt_transfn(PG_FUNCTION_ARGS)
 			ctx->id_name = text_to_cstring(PG_GETARG_TEXT_P(5));
 		else
 			ctx->id_name = NULL;
+
+		ctx->trans_context = AllocSetContextCreate(aggcontext, "MVT transfn", ALLOCSET_DEFAULT_SIZES);
+
+		MemoryContextSwitchTo(ctx->trans_context);
 		mvt_agg_init_context(ctx);
+		MemoryContextSwitchTo(old_context);
 	} else {
 		ctx = (mvt_agg_context *) PG_GETARG_POINTER(0);
 	}
@@ -160,7 +165,10 @@ Datum pgis_asmvt_transfn(PG_FUNCTION_ARGS)
 		elog(ERROR, "%s: parameter row cannot be other than a rowtype", __func__);
 	ctx->row = PG_GETARG_HEAPTUPLEHEADER(1);
 
+	old_context = MemoryContextSwitchTo(ctx->trans_context);
 	mvt_agg_transfn(ctx);
+	MemoryContextSwitchTo(old_context);
+
 	PG_FREE_IF_COPY(ctx->row, 1);
 	PG_RETURN_POINTER(ctx);
 #endif
@@ -203,6 +211,7 @@ Datum pgis_asmvt_serialfn(PG_FUNCTION_ARGS)
 	PG_RETURN_NULL();
 #else
 	mvt_agg_context *ctx;
+	bytea *result;
 	elog(DEBUG2, "%s called", __func__);
 	if (!AggCheckCallContext(fcinfo, NULL))
 		elog(ERROR, "%s called in non-aggregate context", __func__);
@@ -215,7 +224,11 @@ Datum pgis_asmvt_serialfn(PG_FUNCTION_ARGS)
 	}
 
 	ctx = (mvt_agg_context *) PG_GETARG_POINTER(0);
-	PG_RETURN_BYTEA_P(mvt_ctx_serialize(ctx));
+	result = mvt_ctx_serialize(ctx);
+	if (ctx->trans_context)
+		MemoryContextDelete(ctx->trans_context);
+	ctx->trans_context = NULL;
+	PG_RETURN_BYTEA_P(result);
 #endif
 }