diff --git a/lib/dml/moodle_read_slave_trait.php b/lib/dml/moodle_read_slave_trait.php
index 2e99699d9de..7f06e5efb8d 100644
--- a/lib/dml/moodle_read_slave_trait.php
+++ b/lib/dml/moodle_read_slave_trait.php
@@ -272,6 +272,25 @@ trait moodle_read_slave_trait {
         $this->select_db_handle($type, $sql);
     }
 
+    /**
+     * This should be called immediately after each db query. It does a clean up of resources.
+     *
+     * @param mixed $result The db specific result obtained from running a query.
+     * @return void
+     */
+    protected function query_end($result) {
+        if ($this->written) {
+            // Adjust the written time.
+            array_walk($this->written, function (&$val) {
+                if ($val === true) {
+                    $val = microtime(true);
+                }
+            });
+        }
+
+        parent::query_end($result);
+    }
+
     /**
      * Select appropriate db handle - readwrite or readonly
      * @param int $type type of query
@@ -323,15 +342,7 @@ trait moodle_read_slave_trait {
 
                     if (isset($this->written[$tablename])) {
                         $now = $now ?: microtime(true);
-                        // Paranoid check.
-                        if ($this->written[$tablename] === true) {
-                            debugging(
-                                "$tablename last written set to true outside transaction - should not happen!",
-                                DEBUG_DEVELOPER
-                            );
-                            $this->written[$tablename] = $now;
-                            return false;
-                        }
+
                         if ($now - $this->written[$tablename] < $this->slavelatency) {
                             return false;
                         }
@@ -342,9 +353,8 @@ trait moodle_read_slave_trait {
                 return true;
             case SQL_QUERY_INSERT:
             case SQL_QUERY_UPDATE:
-                $now = $this->transactions ? true : microtime(true);
                 foreach ($this->table_names($sql) as $tablename) {
-                    $this->written[$tablename] = $now;
+                    $this->written[$tablename] = true;
                 }
                 return false;
             case SQL_QUERY_STRUCTURE:
diff --git a/lib/dml/pgsql_native_moodle_database.php b/lib/dml/pgsql_native_moodle_database.php
index dd5095002c4..81143e3f53c 100644
--- a/lib/dml/pgsql_native_moodle_database.php
+++ b/lib/dml/pgsql_native_moodle_database.php
@@ -41,6 +41,7 @@ class pgsql_native_moodle_database extends moodle_database {
         select_db_handle as read_slave_select_db_handle;
         can_use_readonly as read_slave_can_use_readonly;
         query_start as read_slave_query_start;
+        query_end as read_slave_query_end;
     }
 
     /** @var array $dbhcursor keep track of open cursors */
@@ -184,12 +185,20 @@ class pgsql_native_moodle_database extends moodle_database {
         }
 
         ob_start();
-        if (empty($this->dboptions['dbpersist'])) {
-            $this->pgsql = pg_connect($connection, PGSQL_CONNECT_FORCE_NEW);
-        } else {
-            $this->pgsql = pg_pconnect($connection, PGSQL_CONNECT_FORCE_NEW);
+        // It seems that pg_connect() handles some errors differently.
+        // For example, name resolution error will raise an exception, and non-existing
+        // database or wrong credentials will just return false.
+        // We need to cater for both.
+        try {
+            if (empty($this->dboptions['dbpersist'])) {
+                $this->pgsql = pg_connect($connection, PGSQL_CONNECT_FORCE_NEW);
+            } else {
+                $this->pgsql = pg_pconnect($connection, PGSQL_CONNECT_FORCE_NEW);
+            }
+            $dberr = ob_get_contents();
+        } catch (\Exception $e) {
+            $dberr = $e->getMessage();
         }
-        $dberr = ob_get_contents();
         ob_end_clean();
 
         $status = $this->pgsql ? pg_connection_status($this->pgsql) : false;
@@ -326,7 +335,7 @@ class pgsql_native_moodle_database extends moodle_database {
         // reset original debug level
         error_reporting($this->last_error_reporting);
         try {
-            parent::query_end($result);
+            $this->read_slave_query_end($result);
             if ($this->savepointpresent and $this->last_type != SQL_QUERY_AUX and $this->last_type != SQL_QUERY_SELECT) {
                 $res = @pg_query($this->pgsql, "RELEASE SAVEPOINT moodle_pg_savepoint; SAVEPOINT moodle_pg_savepoint");
                 if ($res) {
diff --git a/lib/dml/tests/dml_mysqli_read_slave_test.php b/lib/dml/tests/dml_mysqli_read_slave_test.php
index 60924169de4..e415957a0d8 100644
--- a/lib/dml/tests/dml_mysqli_read_slave_test.php
+++ b/lib/dml/tests/dml_mysqli_read_slave_test.php
@@ -21,7 +21,6 @@
  * @category   dml
  * @copyright  2018 Srdjan Janković, Catalyst IT
  * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
- * @coversDefaultClass \mysqli_native_moodle_database
  */
 
 namespace core;
@@ -37,6 +36,7 @@ require_once(__DIR__.'/fixtures/read_slave_moodle_database_mock_mysqli.php');
  * @category   dml
  * @copyright  2018 Catalyst IT
  * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ * @covers     \mysqli_native_moodle_database
  */
 class dml_mysqli_read_slave_test extends \base_testcase {
     /**
diff --git a/lib/dml/tests/dml_pgsql_read_slave_test.php b/lib/dml/tests/dml_pgsql_read_slave_test.php
index 1ad15bf77f8..d98b51a83d3 100644
--- a/lib/dml/tests/dml_pgsql_read_slave_test.php
+++ b/lib/dml/tests/dml_pgsql_read_slave_test.php
@@ -21,7 +21,6 @@
  * @category   dml
  * @copyright  2018 Srdjan Janković, Catalyst IT
  * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
- * @coversDefaultClass \pgsql_native_moodle_database
  */
 
 namespace core;
@@ -37,6 +36,7 @@ require_once(__DIR__.'/fixtures/read_slave_moodle_database_mock_pgsql.php');
  * @category   dml
  * @copyright  2018 Catalyst IT
  * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ * @covers     \pgsql_native_moodle_database
  */
 class dml_pgsql_read_slave_test extends \base_testcase {
     /**
diff --git a/lib/dml/tests/dml_read_slave_test.php b/lib/dml/tests/dml_read_slave_test.php
index f9b5b070a94..88e294e26a6 100644
--- a/lib/dml/tests/dml_read_slave_test.php
+++ b/lib/dml/tests/dml_read_slave_test.php
@@ -21,7 +21,6 @@
  * @category   dml
  * @copyright  2018 Srdjan Janković, Catalyst IT
  * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
- * @coversDefaultClass \moodle_temptables
  */
 
 namespace core;
@@ -39,13 +38,12 @@ require_once(__DIR__.'/../../tests/fixtures/event_fixtures.php');
  * @category   dml
  * @copyright  2018 Catalyst IT
  * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ * @covers     \moodle_read_slave_trait
  */
 class dml_read_slave_test extends \base_testcase {
 
     /** @var float */
     static private $dbreadonlylatency = 0.8;
-    /** @var float */
-    static private $defaultlatency = 1;
 
     /**
      * Instantiates a test database interface object.
@@ -335,6 +333,50 @@ class dml_read_slave_test extends \base_testcase {
         }
     }
 
+    /**
+     * Test readonly handle is not used immediately after update
+     * Test last written time is adjusted post-write,
+     * so the latency parameter is applied properly.
+     *
+     * @return void
+     * @covers ::can_use_readonly
+     * @covers ::query_end
+     */
+    public function test_long_update(): void {
+        $DB = $this->new_db(true);
+
+        $this->assertNull($DB->get_dbhwrite());
+
+        $skip = false;
+
+        list($sql, $params, $ptype) = $DB->fix_sql_params("UPDATE {table} SET a = 1 WHERE id = 1");
+        $DB->with_query_start_end($sql, $params, SQL_QUERY_UPDATE, function ($dbh) use (&$now) {
+            sleep(1);
+            $now = microtime(true);
+        });
+
+        // This condition should always evaluate true, however we need to
+        // safeguard from an unaccounted delay that can break this test.
+        if (microtime(true) - $now < self::$dbreadonlylatency) {
+            // Not enough time passed, use rw handle.
+            $handle = $DB->get_records_sql("SELECT * FROM {table}");
+            $this->assertEquals('test_rw::test:test', $handle);
+
+            // Make sure enough time passes.
+            sleep(1);
+        } else {
+            $skip = true;
+        }
+
+        // Exceeded latency time, use ro handle.
+        $handle = $DB->get_records_sql("SELECT * FROM {table}");
+        $this->assert_readonly_handle($handle);
+
+        if ($skip) {
+            $this->markTestSkipped("Delay too long to test write handle immediately after transaction");
+        }
+    }
+
     /**
      * Test readonly handle is not used with events
      * when the latency parameter is applied properly.
diff --git a/lib/dml/tests/fixtures/read_slave_moodle_database.php b/lib/dml/tests/fixtures/read_slave_moodle_database.php
index db41bd977d2..7659854c7b3 100644
--- a/lib/dml/tests/fixtures/read_slave_moodle_database.php
+++ b/lib/dml/tests/fixtures/read_slave_moodle_database.php
@@ -93,11 +93,15 @@ class read_slave_moodle_database extends test_moodle_database {
      * @param string $sql
      * @param array $params
      * @param int $querytype
+     * @param ?callable $callback
      * @return string $handle handle property
      */
-    private function with_query_start_end($sql, array $params = null, $querytype) {
+    public function with_query_start_end($sql, array $params = null, $querytype, $callback = null) {
         $this->query_start($sql, $params, $querytype);
         $ret = $this->handle;
+        if ($callback) {
+            call_user_func($callback, $ret);
+        }
         $this->query_end(null);
         return $ret;
     }
diff --git a/lib/dml/tests/fixtures/test_moodle_read_slave_trait.php b/lib/dml/tests/fixtures/test_moodle_read_slave_trait.php
index 087e6e94f4c..6172c3c174e 100644
--- a/lib/dml/tests/fixtures/test_moodle_read_slave_trait.php
+++ b/lib/dml/tests/fixtures/test_moodle_read_slave_trait.php
@@ -48,6 +48,7 @@ trait test_moodle_read_slave_trait {
         $ro = fopen("php://memory", 'r+');
         fputs($ro, 'ro');
 
+        $this->prefix = 'test_'; // Default, not to leave empty.
         $this->wantreadslave = true;
         $this->dbhwrite = $rw;
         $this->dbhreadonly = $ro;
@@ -107,6 +108,7 @@ trait test_moodle_read_slave_trait {
      * @param mixed $result
      */
     public function query_end($result) {
+        parent::query_end($result);
         $this->set_db_handle($this->dbhwrite);
     }