Cache IsExtensionRegistered

Doing a btree lookup in `pg_extension` for every query is not free, and
an extension introducing one can (based on my experience) impact
performance for single row lookups significantly.

This introduces a cache which tracks if the extension is registered. I
plan to use this cache in later PRs for more things, such as oids of our
DuckDB functions.

This cache is invalidated whenever an invalidation message is sent for
the "duckdb" schema, which just so happens to be whenever `CREATE/DROP
EXTENSION pg_duckdb` is executed. These invalidation messages are the
mechanism that Postgres uses to let other backends know that some DDL
has occured. We'll probably want to do something else for `ALTER
EXTENSION pg_duckdb UPDATE`, but for now this is good enough (since
there's no upgrades yet). It would have been best if we receive
invalidation messages for rows in `pg_extension`. But sadly this is not
possible. I submitted a patch to upstream Postgres to hopefully allow
for this in the future (but that will only be in PG18+):
https://www.postgresql.org/message-id/flat/CAGECzQTWm9sex719Hptbq4j56hBGUti7J9OWjeMobQ1ccRok9w%40mail.gmail.com

Makefile

@@ -10,6 +10,7 @@ SRCS = src/scan/heap_reader.cpp \
 	   src/scan/postgres_scan.cpp \
 	   src/scan/postgres_seq_scan.cpp \
 	   src/utility/copy.cpp \
+	   src/pgduckdb_cache.cpp \
 	   src/pgduckdb_detoast.cpp \
 	   src/pgduckdb_duckdb.cpp \
 	   src/pgduckdb_filter.cpp \

include/pgduckdb/pgduckdb_cache.hpp

@@ -0,0 +1,3 @@
+namespace pgduckdb {
+bool IsExtensionRegistered();
+}

src/pgduckdb_cache.cpp

@@ -0,0 +1,45 @@
+extern "C" {
+#include "postgres.h"
+
+#include "commands/extension.h"
+#include "utils/inval.h"
+#include "utils/syscache.h"
+}
+
+namespace pgduckdb {
+struct {
+	bool valid;
+	bool installed;
+} cache = {};
+
+bool callback_is_configured = false;
+uint32 schema_hash_value;
+
+static void
+InvalidateCaches(Datum arg, int cache_id, uint32 hash_value) {
+	if (hash_value != schema_hash_value) {
+		return;
+	}
+	cache.valid = false;
+}
+
+bool
+IsExtensionRegistered() {
+	if (cache.valid) {
+		return cache.installed;
+	}
+
+	cache.installed = get_extension_oid("pg_duckdb", true) != InvalidOid;
+	cache.valid = true;
+
+	if (!callback_is_configured) {
+		callback_is_configured = true;
+		schema_hash_value = GetSysCacheHashValue1(NAMESPACENAME, CStringGetDatum("duckdb"));
+
+		CacheRegisterSyscacheCallback(NAMESPACENAME, InvalidateCaches, (Datum)0);
+	}
+
+	return cache.installed;
+}
+
+} // namespace pgduckdb

src/pgduckdb_hooks.cpp

@@ -10,18 +10,14 @@ extern "C" {
 }
 
 #include "pgduckdb/pgduckdb.h"
+#include "pgduckdb/pgduckdb_cache.hpp"
 #include "pgduckdb/pgduckdb_planner.hpp"
 #include "pgduckdb/utility/copy.hpp"
 #include "pgduckdb/vendor/pg_list.hpp"
 
 static planner_hook_type prev_planner_hook = NULL;
 static ProcessUtility_hook_type prev_process_utility_hook = NULL;
 
-static bool
-IsDuckdbExtensionRegistered() {
-	return get_extension_oid("pg_duckdb", true) != InvalidOid;
-}
-
 static bool
 IsCatalogTable(List *tables) {
 	foreach_node(RangeTblEntry, table, tables) {
@@ -57,7 +53,7 @@ IsAllowedStatement() {
 
 static PlannedStmt *
 DuckdbPlannerHook(Query *parse, const char *query_string, int cursor_options, ParamListInfo bound_params) {
-	if (duckdb_execution && IsAllowedStatement() && IsDuckdbExtensionRegistered() && parse->rtable &&
+	if (duckdb_execution && IsAllowedStatement() && pgduckdb::IsExtensionRegistered() && parse->rtable &&
 	    !IsCatalogTable(parse->rtable) && parse->commandType == CMD_SELECT) {
 		PlannedStmt *duckdb_plan = DuckdbPlanNode(parse, query_string, cursor_options, bound_params);
 		if (duckdb_plan) {
@@ -76,7 +72,7 @@ static void
 DuckdbUtilityHook(PlannedStmt *pstmt, const char *query_string, bool read_only_tree, ProcessUtilityContext context,
                   ParamListInfo params, struct QueryEnvironment *query_env, DestReceiver *dest, QueryCompletion *qc) {
 	Node *parsetree = pstmt->utilityStmt;
-	if (duckdb_execution && IsDuckdbExtensionRegistered() && IsA(parsetree, CopyStmt)) {
+	if (duckdb_execution && pgduckdb::IsExtensionRegistered() && IsA(parsetree, CopyStmt)) {
 		uint64 processed;
 		if (DuckdbCopy(pstmt, query_string, query_env, &processed)) {
 			if (qc) {