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[Robert Lyon <robertl@xxxxxxxxxxxxxxx>]

Actually, looking at the code I have in the migration script, it looks
like it's meant to sort out artefact hierarchy - so I'll add it here:

        // need to do special sortby as child artefacts can have a lower id number than their parent
        // due to one being able to move older files into newer folders
        if (is_postgres()) {
            $sourcedata = get_recordset_sql("; WITH RECURSIVE cte AS
                (
                 SELECT *, id AS parentid, 1 AS hierarchy FROM artefact
                 WHERE parent IS NULL
                 UNION ALL
                 SELECT t.*, cte.parentid, hierarchy + 1 FROM artefact t INNER JOIN cte ON t.parent = cte.id
                ) SELECT * FROM cte ORDER BY hierarchy");
        }
        else if(is_mysql()) {
            // we need to do a trick as mysql doesn't have WITH RECURSIVE option
            // 1) we need to add the WITH_EMULATOR proceedure to mysql database
            // See the mysqli_with_recursive_alternative.txt in this dir

            // 2) we call the prepared staement
            $sourcedata = get_recordset_sql('CALL WITH_EMULATOR(\'cte\', \'SELECT *, id AS parentid, 1 AS hierarchy FROM artefact WHERE parent IS NULL\',\'SELECT t.*, cte.parentid, hierarchy + 1 FROM artefact t INNER JOIN cte ON t.parent = cte.id\',\'SELECT * FROM cte ORDER BY hierarchy\', 0, \'\')');
        }

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https://bugs.launchpad.net/bugs/1427885

Title:
  Change "artefact.path" column to use the "nested set" technique for
  managing hierarchical data

Status in Mahara ePortfolio:
  Triaged

Bug description:
  Originally, we just had each artefact store its parent ID. This is
  slow because it requires running multiple queries to find all the
  descendants of a node.

  Then, we added a "path" element to each artefact. This is better, but
  you can't get a performance improvement by indexing the column,
  because most of the queries rely on the "LIKE" operator. (See
  https://bugs.launchpad.net/mahara/+bug/1423700 )

  So if we want to squeeze more performance out of this, I think the one
  remaining thing to look into is the "nested set" technique. This
  technique results in very fast searches for descendants, with the cost
  of somewhat slower writes. http://mikehillyer.com/articles/managing-
  hierarchical-data-in-mysql/

  There are even existing PHP libraries for using the technique, such as
  this one: http://www.sideralis.org/baobab/

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