Do you only update the changed fields or all the fields?

Question

I'm wondering if it's worth the server time when updating a record to retrieve the existing record, loop through the fields checking for changes and only putting the changed fields in the update query? (I am using MySQL & PHP.)

The main reason for doing this is to reduce the size of the update query for change log purposes. Normally the query might have 15 fields, but only 2 fields are actually being changed. This query can then also be used for logging as it will only contain the changed fields and therefore is easier to parse.

My concern is the time that it takes to retrieve the existing record.

Or is there a way to retrieve from MySQL which fields it updated?

Answer 1

I think it's worth changing - but probably not worth doing a select before insert.

I only update the fields that have changed, it's part of the operation of my DbEntity class which follows an activerecord pattern. It costs little extra to do this because I hold the current record and original records -simply copying whenever a record is loaded.

Reasons are brevity - not really performance. Also you can check for concurrent modification by adding a where clause on the old value of the updated fields and throw the appropriate error.

In the write/update method:

$s1 = "";

foreach ($this->record as $key => $value)
{
    // only update fields that have been changed
    if ($value != $this->orig_record[$key])
    {
        $s1 .= $comma."`$key`='".mysql_real_escape_string($value)."'";
        $comma = ", ";
    }
}

$query = "UPDATE ".$this->table." SET $s1 where {$this->id_field}='".$this->get_keyfield()."'";
$query .= $this->extra_sql_update;
mysql_query($query);

$ar = mysql_affected_rows();
//
// the number of affected rows is actually those changed by the update operation, which will 
// either be zero, or 1. If the query affects more than one row then we have a problem.
if ($ar < 0 || $ar > 1)
{
    cbf_error("cbf_dbentity: {$this->table} :: only one row (not $ar) must be affected by an insert operation. $query",
      E_USER_ERROR);
}
else
{
    $new_id = $this->get_keyfield();

    GlobalEventBus::notify_all(new AuditLogSQL($this->table, "update", $query));

}

$this->orig_record = Array();

foreach ($this->record as $key => $value)
    $this->orig_record[$key] = $value;


//
// sanity check - ensure that what we have just written is actually there.

$this->load($new_id);

foreach ($this->orig_record as $key => $value)
    if (trim($this->record[$key]) != trim($value) 
        && (!$this->record[$key] == "0" && $value=""))
        cbf_error("cbf_dbentity: {$this->table} :: record differs during write after reload: field $key was \"$value\", after write it is now \"".
              $this->record[$key]."\"",E_USER_ERROR);

In the load method

$this->orig_record = Array();
foreach ($this->record as $key => $value)
    $this->orig_record[$key] = $value;