SQL Repository - problematic queries
Last modified 11 May 2021 08:37 +02:00
Table of Contents

This document identifes problematic queries existing in 4.0-4.2 versions and ideas what to do with them. The queries may not be critical in smaller deployments, but proved to be in large scale ones.

Partitioned validity scanner task

  • Symptoms: extremely high disk IO

  • Called occassionally

  • Database: Oracle

  • Known fact: JOIN to m_focus and m_user is useless, m_user.oid can be replaced with m_object.oid. (Hibernate in Action)

  • Questions: What is the count of returned rows when the problem occures?

SELECT
ruser0_.oid            AS col_0_0_,
ruser0_2_.fullobject   AS col_1_0_
FROM m_user ruser0_
INNER JOIN m_focus ruser0_1_ ON ruser0_.oid = ruser0_1_.oid
INNER JOIN m_object ruser0_2_ ON ruser0_.oid = ruser0_2_.oid
LEFT OUTER JOIN m_assignment assignment1_
  ON ruser0_.oid = assignment1_.owner_oid AND ( assignment1_.assignmentowner =:1 )
WHERE assignment1_.validfrom >:2
  AND assignment1_.validfrom <=:3
OR assignment1_.validto >:4
  AND assignment1_.validto <=:5
order by nlssort(ruser0_.oid,'NLS_SORT=BINARY_AI') asc
fetch first :6 rows only

  • Both validfrom and validto are indexed and the indexes should be used. Both (date > x and date ⇐ y) should result in FILTER with INDEX RANGE SCAN. This can be good depending on the range of the scan.

  • Both clauses are joined with OR which internally performs UNION-ALL with the two FILTER results.

  • ORDER BY on experimental tables (without any joins, just oid + two timestamps) shows much higher "Total cost" than both filters. E.g., with 2-3k of rows order cost is 72, filter cost is 7+3 with over 100k total rows. The tenfold factor stays roughly the same with 13k rows of result.

  • Also, actual select uses function in ORDER, the question is whether function based index could help.

  • Assignment table is very big (tens of millions rows and more).

In the new repo, this should be more like (PostgreSQL style):

SELECT u.oid, u.fullobject FROM m_user u
WHERE EXISTS (
    SELECT * FROM m_assignment a
        WHERE a.owner_oid = u.oid -- "join", this should be super efficient
            AND a.assignmentowner = ? -- discriminator, perhaps not necessary for user
            AND (a.validfrom > ? AND a.validfrom <= ?
                OR a.validto > ? AND validto <= ? ))
ORDER BY u.oid
LIMIT ?

  • It’s good it’s limited, this in general is one of the biggest performance boosts. Count queries can’t benefit from this and can take disproportionately longer for the same query.

  • Assignments conditions are in subquery now, this also leaves the outer query with the right number of rows.

  • No abstract table joins needed, all information is in a single table.

The same problem as above, but obviously without order this time.

SELECT
COUNT(ruser0_.oid) AS col_0_0_
FROM m_user ruser0_
INNER JOIN m_focus ruser0_1_ ON ruser0_.oid = ruser0_1_.oid
INNER JOIN m_object ruser0_2_ ON ruser0_.oid = ruser0_2_.oid
LEFT OUTER JOIN m_assignment assignment1_
  ON ruser0_.oid = assignment1_.owner_oid AND ( assignment1_.assignmentowner =:1 )
WHERE assignment1_.validfrom >:2
  AND assignment1_.validfrom <=:3
OR assignment1_.validto >:4
  AND assignment1_.validto <=:5

  • Called very often.

  • Cause of slowness seems to be the sheer amount of shadows and their string extensions.

  • This may be improved by partitioning by objectclass and resourceref_targetoid, which otherwise have very low variability for a normal (b-tree) index.

  • FK m_object_ext_string.owner_oid is coverd by PK index.

SELECT
rshadow0_.oid            AS col_0_0_,
rshadow0_1_.fullobject   AS col_1_0_
FROM
m_shadow rshadow0_
INNER JOIN m_object rshadow0_1_ ON rshadow0_.oid = rshadow0_1_.oid
LEFT OUTER JOIN m_object_ext_string strings1_
  ON rshadow0_.oid = strings1_.owner_oid -- covered by PK, owner_oid is first there
    AND ( strings1_.ownertype =:1 AND strings1_.item_id =:2 )
WHERE strings1_.stringvalue =:3
AND rshadow0_.objectclass =:4
AND rshadow0_.resourceref_targetoid =:5
AND ( rshadow0_.resourceref_relation IN ( :6, :7, :8 ) )
AND ( rshadow0_.dead =:9 OR rshadow0_.dead IS NULL )
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