[#28894] docdb: Enable skip prefix locks by default

Summary:
Set skip_prefix_locks = true to enable skip prefix locks by default.
Jira: DB-18616

Test Plan: existing unit tests

Reviewers: rthallam

Reviewed By: rthallam

Subscribers: ybase, yql

Differential Revision: https://phorge.dev.yugabyte.com/D47359

Author: huapengy

java/yb-pgsql/src/test/java/org/yb/pgsql/TestPgRegressProc.java

@@ -34,6 +34,7 @@ protected Map<String, String> getTServerFlags() {
     Map<String, String> flagMap = super.getTServerFlags();
     // Setting the below flag stabilizes yb_lock_status tests.
     flagMap.put("TEST_delay_before_get_old_transactions_heartbeat_intervals", "2");
+    flagMap.put("skip_prefix_locks", "false");
     return flagMap;
   }
 

src/yb/tablet/write_query.cc

@@ -64,12 +64,17 @@ DEFINE_RUNTIME_bool(disable_alter_vs_write_mutual_exclusion, false,
     "operation take an exclusive lock making all write operations wait for it.");
 TAG_FLAG(disable_alter_vs_write_mutual_exclusion, advanced);
 
-DEFINE_RUNTIME_bool(skip_prefix_locks, false,
+DEFINE_RUNTIME_bool(skip_prefix_locks, true,
     "Enable to skip writing weak intent locks on intermediate prefixes of the actual PK being "
     "written. This reduces the number of key-value pairs written to the intents db, resulting in "
     "better performance. However, it may lead to high contention when serializable isolation "
     "transactions are present in the workload.");
 
+DEFINE_RUNTIME_AUTO_bool(skip_prefix_locks_for_upgrade, kLocalPersisted, false, true,
+    "The gflag is for upgrade and should not be changed. skip prefix lock is enabled only if "
+    "both skip_prefix_locks and skip_prefix_locks_for_upgrade are true. To disable the feature, "
+    "just set skip_prefix_locks to false.");
+
 DEFINE_RUNTIME_bool(skip_prefix_locks_suppress_warning_for_serializable_op, false,
     "If false, logs a warning when a serializable transaction runs with skip prefix locks enabled."
     "Serializable isolation transactions are not meant to be run if skip_prefix_locks is enabled. "
@@ -830,7 +835,8 @@ Status WriteQuery::DoExecute() {
     auto transaction_id = VERIFY_RESULT(FullyDecodeTransactionId(
       write_batch.transaction().transaction_id()));
 
-    const bool should_skip_prefix_locks = FLAGS_skip_prefix_locks && FLAGS_ysql_enable_packed_row;
+    const bool should_skip_prefix_locks = FLAGS_skip_prefix_locks &&
+        FLAGS_skip_prefix_locks_for_upgrade && FLAGS_ysql_enable_packed_row;
     skip_prefix_locks = should_skip_prefix_locks ?
         dockv::SkipPrefixLocks::kTrue : dockv::SkipPrefixLocks::kFalse;
     write_batch.mutable_transaction()->set_skip_prefix_locks(should_skip_prefix_locks);