From 17da93d42bcb26b8ac770d1a52142ed1dc0d3620 Mon Sep 17 00:00:00 2001
From: Andrew Nicols <andrew@nicols.co.uk>
Date: Thu, 8 Aug 2019 14:03:41 +0800
Subject: [PATCH] MDL-66614 pgsql DML: Update get_records_sql to consume less
 memory

This is a backport of MDL-66327.

It's more memory efficient to use `pg_fetch_assoc` for each row than to
call `pg_fetch_all` and release memory immediately. This is because we
can treat the assoc fetch like an iterator and it only fetches the
current record into memory one at a time, whilst the all fetch fetches
all records and never unsets them. Attempting to unset them is extremely
time consuming.
---
 lib/dml/pgsql_native_moodle_database.php | 30 ++++++++++--------------
 1 file changed, 12 insertions(+), 18 deletions(-)

diff --git a/lib/dml/pgsql_native_moodle_database.php b/lib/dml/pgsql_native_moodle_database.php
index 97d94905ff9..d894b366070 100644
--- a/lib/dml/pgsql_native_moodle_database.php
+++ b/lib/dml/pgsql_native_moodle_database.php
@@ -842,8 +842,7 @@ class pgsql_native_moodle_database extends moodle_database {
      * @return array of objects, or empty array if no records were found
      * @throws dml_exception A DML specific exception is thrown for any errors.
      */
-    public function get_records_sql($sql, array $params=null, $limitfrom=0, $limitnum=0) {
-
+    public function get_records_sql($sql, array $params = null, $limitfrom = 0, $limitnum = 0) {
         list($limitfrom, $limitnum) = $this->normalise_limit_from_num($limitfrom, $limitnum);
 
         if ($limitnum) {
@@ -868,24 +867,19 @@ class pgsql_native_moodle_database extends moodle_database {
             }
         }
 
-        $rows = pg_fetch_all($result);
-        pg_free_result($result);
-
-        $return = array();
-        if ($rows) {
-            foreach ($rows as $row) {
-                $id = reset($row);
-                if ($blobs) {
-                    foreach ($blobs as $blob) {
-                        $row[$blob] = ($row[$blob] !== null ? pg_unescape_bytea($row[$blob]) : null);
-                    }
+        $return = [];
+        while ($row = pg_fetch_assoc($result)) {
+            $id = reset($row);
+            if ($blobs) {
+                foreach ($blobs as $blob) {
+                    $row[$blob] = ($row[$blob] !== null ? pg_unescape_bytea($row[$blob]) : null);
                 }
-                if (isset($return[$id])) {
-                    $colname = key($row);
-                    debugging("Did you remember to make the first column something unique in your call to get_records? Duplicate value '$id' found in column '$colname'.", DEBUG_DEVELOPER);
-                }
-                $return[$id] = (object)$row;
             }
+            if (isset($return[$id])) {
+                $colname = key($row);
+                debugging("Did you remember to make the first column something unique in your call to get_records? Duplicate value '$id' found in column '$colname'.", DEBUG_DEVELOPER);
+            }
+            $return[$id] = (object) $row;
         }
 
         return $return;