MDL-49482 mssql: Implement support of Offset/Fetch in SQL Server 2012+.

lib/dml/mssql_native_moodle_database.php

@@ -40,6 +40,11 @@ class mssql_native_moodle_database extends moodle_database {
     protected $mssql     = null;
     protected $last_error_reporting; // To handle mssql driver default verbosity
     protected $collation;  // current DB collation cache
+    /**
+     * Does the used db version support ANSI way of limiting (2012 and higher)
+     * @var bool
+     */
+    protected $supportsoffsetfetch;
 
     /**
      * Detects if all needed PHP stuff installed.
@@ -229,6 +234,10 @@ class mssql_native_moodle_database extends moodle_database {
 
         $this->free_result($result);
 
+        $serverinfo = $this->get_server_info();
+        // Fetch/offset is supported staring from SQL Server 2012.
+        $this->supportsoffsetfetch = $serverinfo['version'] > '11';
+
         // Connection stabilised and configured, going to instantiate the temptables controller
         $this->temptables = new mssql_native_moodle_temptables($this);
 
@@ -737,13 +746,28 @@ class mssql_native_moodle_database extends moodle_database {
         list($limitfrom, $limitnum) = $this->normalise_limit_from_num($limitfrom, $limitnum);
 
         if ($limitfrom or $limitnum) {
-            if ($limitnum >= 1) { // Only apply TOP clause if we have any limitnum (limitfrom offset is handled later)
-                $fetch = $limitfrom + $limitnum;
-                if (PHP_INT_MAX - $limitnum < $limitfrom) { // Check PHP_INT_MAX overflow
-                    $fetch = PHP_INT_MAX;
+            if (!$this->supportsoffsetfetch) {
+                if ($limitnum >= 1) { // Only apply TOP clause if we have any limitnum (limitfrom offset is handled later).
+                    $fetch = $limitfrom + $limitnum;
+                    if (PHP_INT_MAX - $limitnum < $limitfrom) { // Check PHP_INT_MAX overflow.
+                        $fetch = PHP_INT_MAX;
+                    }
+                    $sql = preg_replace('/^([\s(])*SELECT([\s]+(DISTINCT|ALL))?(?!\s*TOP\s*\()/i',
+                                        "\\1SELECT\\2 TOP $fetch", $sql);
+                }
+            } else {
+                $sql = (substr($sql, -1) === ';') ? substr($sql, 0, -1) : $sql;
+                // We need order by to use FETCH/OFFSET.
+                // Ordering by first column shouldn't break anything if there was no order in the first place.
+                if (!strpos(strtoupper($sql), "ORDER BY")) {
+                    $sql .= " ORDER BY 1";
+                }
+
+                $sql .= " OFFSET ".$limitfrom." ROWS ";
+
+                if ($limitnum > 0) {
+                    $sql .= " FETCH NEXT ".$limitnum." ROWS ONLY";
                 }
-                $sql = preg_replace('/^([\s(])*SELECT([\s]+(DISTINCT|ALL))?(?!\s*TOP\s*\()/i',
-                                    "\\1SELECT\\2 TOP $fetch", $sql);
             }
         }
 
@@ -754,7 +778,7 @@ class mssql_native_moodle_database extends moodle_database {
         $result = mssql_query($rawsql, $this->mssql);
         $this->query_end($result);
 
-        if ($limitfrom) { // Skip $limitfrom records
+        if ($limitfrom && !$this->supportsoffsetfetch) { // Skip $limitfrom records.
             if (!@mssql_data_seek($result, $limitfrom)) {
                 // Nothing, most probably seek past the end.
                 mssql_free_result($result);