diff --git a/lib/datalib.php b/lib/datalib.php
index 285729c23c3..88066531899 100644
--- a/lib/datalib.php
+++ b/lib/datalib.php
@@ -1710,7 +1710,7 @@ function add_to_log($courseid, $module, $action, $url='', $info='', $cm=0, $user
 
     try {
         $DB->insert_record_raw('log', $log, false);
-    } catch (dml_write_exception $e) {
+    } catch (dml_exception $e) {
         debugging('Error: Could not insert a new entry to the Moodle log', DEBUG_ALL);
         // MDL-11893, alert $CFG->supportemail if insert into log failed
         if ($CFG->supportemail and empty($CFG->noemailever)) {
diff --git a/lib/ddl/simpletest/testddl.php b/lib/ddl/simpletest/testddl.php
index 7ea1275bbd4..a9377534fc9 100644
--- a/lib/ddl/simpletest/testddl.php
+++ b/lib/ddl/simpletest/testddl.php
@@ -180,7 +180,7 @@ class ddl_test extends UnitTestCase {
         ob_start(); // hide debug warning
         try {
             $result = $DB->get_records('test_table0');
-        } catch (dml_read_exception $e) {
+        } catch (dml_exception $e) {
             $result = false;
         }
         ob_end_clean();
@@ -203,7 +203,7 @@ class ddl_test extends UnitTestCase {
         ob_start(); // hide debug warning
         try {
             $result = $DB->get_records('test_table0');
-        } catch (dml_read_exception $e) {
+        } catch (dml_exception $e) {
             $result = false;
         }
         ob_end_clean();
@@ -337,14 +337,14 @@ class ddl_test extends UnitTestCase {
 
         try { // columns cache must be empty, so sentence throw exception
             $columns = $DB->get_columns('test_table0');
-        } catch (dml_read_exception $e) {
+        } catch (dml_exception $e) {
             $columns = false;
         }
         $this->assertFalse($columns);
 
         try { /// throw exception
             $indexes = $DB->get_indexes('test_table0');
-        } catch (dml_read_exception $e) {
+        } catch (dml_exception $e) {
             $indexes = false;
         }
         $this->assertFalse($indexes);
@@ -931,7 +931,7 @@ class ddl_test extends UnitTestCase {
         ob_start(); // hide debug warning
         try {
             $result = $DB->insert_record('test_table_cust0', $record, false);
-        } catch (dml_write_exception $e) {
+        } catch (dml_exception $e) {
             $result = false;
         }
         ob_end_clean();
@@ -953,7 +953,7 @@ class ddl_test extends UnitTestCase {
         ob_start(); // hide debug warning
         try {
             $result = $DB->insert_record('test_table_cust0', $record, false);
-        } catch (dml_write_exception $e) {
+        } catch (dml_exception $e) {
             $result = false;
         }
         ob_end_clean();
@@ -1022,7 +1022,7 @@ class ddl_test extends UnitTestCase {
         ob_start(); // hide debug warning
         try {
             $result = $DB->insert_record('test_table_cust0', $record, false);
-        } catch (dml_write_exception $e) {
+        } catch (dml_exception $e) {
             $result = false;;
         }
         ob_end_clean();
@@ -1644,7 +1644,7 @@ class ddl_test extends UnitTestCase {
 
             $rec = $DB->get_record($tablename, array('id'=>$id));
             $this->assertIdentical($rec->name, $maxstr);
-        } catch (dml_write_exception $e) {
+        } catch (dml_exception $e) {
             if ($DB->get_dbfamily() === 'oracle') {
                 $this->fail('Oracle does not support text fields larger than 4000 bytes, this is not a big problem for mostly ascii based languages');
             } else {
diff --git a/lib/dml/moodle_database.php b/lib/dml/moodle_database.php
index 00bbd96b409..6e00b4c184b 100644
--- a/lib/dml/moodle_database.php
+++ b/lib/dml/moodle_database.php
@@ -495,15 +495,15 @@ abstract class moodle_database {
      * @return array sql part and params
      */
     protected function where_clause($table, array $conditions=null) {
-        $allowed_types = $this->allowed_param_types();
-        if (empty($conditions)) {
-            return array('', array());
-        }
-        $where = array();
-        $params = array();
-
+        // We accept nulls in conditions
+        $conditions = is_null($conditions) ? array() : $conditions;
+        // Some checks performed under debugging only
         if (debugging()) {
             $columns = $this->get_columns($table);
+            if (empty($columns)) {
+                // no supported columns means most probably table does not exist
+                throw new dml_exception('ddltablenotexist', $table);
+            }
             foreach ($conditions as $key=>$value) {
                 if (!isset($columns[$key])) {
                     $a = new stdClass();
@@ -519,6 +519,13 @@ abstract class moodle_database {
             }
         }
 
+        $allowed_types = $this->allowed_param_types();
+        if (empty($conditions)) {
+            return array('', array());
+        }
+        $where = array();
+        $params = array();
+
         foreach ($conditions as $key=>$value) {
             if (is_int($key)) {
                 throw new dml_exception('invalidnumkey');
diff --git a/lib/dml/mysqli_native_moodle_database.php b/lib/dml/mysqli_native_moodle_database.php
index 7b6f2f3d9d4..fabfb3f1b70 100644
--- a/lib/dml/mysqli_native_moodle_database.php
+++ b/lib/dml/mysqli_native_moodle_database.php
@@ -435,7 +435,7 @@ class mysqli_native_moodle_database extends moodle_database {
         $sql = "SHOW COLUMNS FROM {$this->prefix}$table";
         $this->query_start($sql, null, SQL_QUERY_AUX);
         $result = $this->mysqli->query($sql);
-        $this->query_end($result);
+        $this->query_end(true); // Don't want to throw anything here ever. MDL-30147
 
         if ($result === false) {
             return array();
diff --git a/lib/dml/simpletest/testdml.php b/lib/dml/simpletest/testdml.php
index 627c6c14638..fce8f4044f5 100644
--- a/lib/dml/simpletest/testdml.php
+++ b/lib/dml/simpletest/testdml.php
@@ -821,6 +821,10 @@ class dml_test extends UnitTestCase {
 
             $this->assertEqual($next_column->name, $next_field->name);
         }
+
+        // Test get_columns for non-existing table returns empty array. MDL-30147
+        $columns = $DB->get_columns('xxxx');
+        $this->assertEqual(array(), $columns);
     }
 
     public function test_get_manager() {
@@ -905,7 +909,7 @@ class dml_test extends UnitTestCase {
             $DB->execute($sql);
             $this->fail("Expecting an exception, none occurred");
         } catch (Exception $e) {
-            $this->assertTrue($e instanceof dml_write_exception);
+            $this->assertTrue($e instanceof dml_exception);
         }
 
         // update records
@@ -999,7 +1003,10 @@ class dml_test extends UnitTestCase {
         $conditions = array('onetext' => '1');
         try {
             $rs = $DB->get_recordset($tablename, $conditions);
-            $this->fail('An Exception is missing, expected due to equating of text fields');
+            if (debugging()) {
+                // only in debug mode - hopefully all devs test code in debug mode...
+                $this->fail('An Exception is missing, expected due to equating of text fields');
+            }
         } catch (exception $e) {
             $this->assertTrue($e instanceof dml_exception);
             $this->assertEqual($e->errorcode, 'textconditionsnotallowed');
@@ -1256,12 +1263,51 @@ class dml_test extends UnitTestCase {
         $conditions = array('onetext' => '1');
         try {
             $records = $DB->get_records($tablename, $conditions);
-            $this->fail('An Exception is missing, expected due to equating of text fields');
+            if (debugging()) {
+                // only in debug mode - hopefully all devs test code in debug mode...
+                $this->fail('An Exception is missing, expected due to equating of text fields');
+            }
         } catch (exception $e) {
             $this->assertTrue($e instanceof dml_exception);
             $this->assertEqual($e->errorcode, 'textconditionsnotallowed');
         }
 
+        // test get_records passing non-existing table
+        // with params
+        try {
+            $records = $DB->get_records('xxxx', array('id' => 0));
+            $this->fail('An Exception is missing, expected due to query against non-existing table');
+        } catch (exception $e) {
+            $this->assertTrue($e instanceof dml_exception);
+            if (debugging()) {
+                // information for developers only, normal users get general error message
+                $this->assertEqual($e->errorcode, 'ddltablenotexist');
+            }
+        }
+        // and without params
+        try {
+            $records = $DB->get_records('xxxx', array());
+            $this->fail('An Exception is missing, expected due to query against non-existing table');
+        } catch (exception $e) {
+            $this->assertTrue($e instanceof dml_exception);
+            if (debugging()) {
+                // information for developers only, normal users get general error message
+                $this->assertEqual($e->errorcode, 'ddltablenotexist');
+            }
+        }
+
+        // test get_records passing non-existing column
+        try {
+            $records = $DB->get_records($tablename, array('xxxx' => 0));
+            $this->fail('An Exception is missing, expected due to query against non-existing column');
+        } catch (exception $e) {
+            $this->assertTrue($e instanceof dml_exception);
+            if (debugging()) {
+                // information for developers only, normal users get general error message
+                $this->assertEqual($e->errorcode, 'ddlfieldnotexist');
+            }
+        }
+
         // note: delegate limits testing to test_get_records_sql()
     }
 
@@ -1641,7 +1687,10 @@ class dml_test extends UnitTestCase {
         $conditions = array('onetext' => '1');
         try {
             $DB->get_field($tablename, 'course', $conditions);
-            $this->fail('An Exception is missing, expected due to equating of text fields');
+            if (debugging()) {
+                // only in debug mode - hopefully all devs test code in debug mode...
+                $this->fail('An Exception is missing, expected due to equating of text fields');
+            }
         } catch (exception $e) {
             $this->assertTrue($e instanceof dml_exception);
             $this->assertEqual($e->errorcode, 'textconditionsnotallowed');
@@ -1785,7 +1834,7 @@ class dml_test extends UnitTestCase {
         try {
             $DB->insert_record_raw($tablename, array('xxxxx' => 3, 'onechar' => 'bb'));
             $this->assertFail('Exception expected due to invalid column');
-        } catch (dml_write_exception $ex) {
+        } catch (dml_exception $ex) {
             $this->assertTrue(true);
         }
     }
@@ -2494,7 +2543,10 @@ class dml_test extends UnitTestCase {
         $conditions = array('onetext' => '1');
         try {
             $DB->set_field($tablename, 'onechar', 'frog', $conditions);
-            $this->fail('An Exception is missing, expected due to equating of text fields');
+            if (debugging()) {
+                // only in debug mode - hopefully all devs test code in debug mode...
+                $this->fail('An Exception is missing, expected due to equating of text fields');
+            }
         } catch (exception $e) {
             $this->assertTrue($e instanceof dml_exception);
             $this->assertEqual($e->errorcode, 'textconditionsnotallowed');
@@ -2704,7 +2756,10 @@ class dml_test extends UnitTestCase {
         $conditions = array('onetext' => '1');
         try {
             $DB->count_records($tablename, $conditions);
-            $this->fail('An Exception is missing, expected due to equating of text fields');
+            if (debugging()) {
+                // only in debug mode - hopefully all devs test code in debug mode...
+                $this->fail('An Exception is missing, expected due to equating of text fields');
+            }
         } catch (exception $e) {
             $this->assertTrue($e instanceof dml_exception);
             $this->assertEqual($e->errorcode, 'textconditionsnotallowed');
@@ -2779,7 +2834,10 @@ class dml_test extends UnitTestCase {
         $conditions = array('onetext' => '1');
         try {
             $DB->record_exists($tablename, $conditions);
-            $this->fail('An Exception is missing, expected due to equating of text fields');
+            if (debugging()) {
+                // only in debug mode - hopefully all devs test code in debug mode...
+                $this->fail('An Exception is missing, expected due to equating of text fields');
+            }
         } catch (exception $e) {
             $this->assertTrue($e instanceof dml_exception);
             $this->assertEqual($e->errorcode, 'textconditionsnotallowed');
@@ -2927,7 +2985,10 @@ class dml_test extends UnitTestCase {
         $conditions = array('onetext'=>'1');
         try {
             $DB->delete_records($tablename, $conditions);
-            $this->fail('An Exception is missing, expected due to equating of text fields');
+            if (debugging()) {
+                // only in debug mode - hopefully all devs test code in debug mode...
+                $this->fail('An Exception is missing, expected due to equating of text fields');
+            }
         } catch (exception $e) {
             $this->assertTrue($e instanceof dml_exception);
             $this->assertEqual($e->errorcode, 'textconditionsnotallowed');
@@ -2937,7 +2998,10 @@ class dml_test extends UnitTestCase {
         $conditions = array('onetext' => 1);
         try {
             $DB->delete_records($tablename, $conditions);
-            $this->fail('An Exception is missing, expected due to equating of text fields');
+            if (debugging()) {
+                // only in debug mode - hopefully all devs test code in debug mode...
+                $this->fail('An Exception is missing, expected due to equating of text fields');
+            }
         } catch (exception $e) {
             $this->assertTrue($e instanceof dml_exception);
             $this->assertEqual($e->errorcode, 'textconditionsnotallowed');
diff --git a/lib/dmllib.php b/lib/dmllib.php
index 19546aec986..2e5945463b5 100644
--- a/lib/dmllib.php
+++ b/lib/dmllib.php
@@ -91,7 +91,7 @@ class dml_sessionwait_exception extends dml_exception {
 }
 
 /**
- * DML read exception - triggered by SQL syntax errors, missing tables, etc.
+ * DML read exception - triggered by some SQL syntax errors, etc.
  */
 class dml_read_exception extends dml_exception {
     /** @var string */
@@ -185,7 +185,7 @@ class dml_missing_record_exception extends dml_exception {
 }
 
 /**
- * DML write exception - triggered by SQL syntax errors, missing tables, etc.
+ * DML write exception - triggered by some SQL syntax errors, etc.
  */
 class dml_write_exception extends dml_exception {
     /** @var string */
diff --git a/lib/setuplib.php b/lib/setuplib.php
index bdb287d50dd..205cd3229e7 100644
--- a/lib/setuplib.php
+++ b/lib/setuplib.php
@@ -657,7 +657,7 @@ function initialise_cfg() {
                 $CFG->{$name} = $value;
             }
         }
-    } catch (dml_read_exception $e) {
+    } catch (dml_exception $e) {
         // most probably empty db, going to install soon
     }
 }