Question

I am trying to optimize a Crystal Report that is used very frequently here. I succeeded to optimize lots of queries but I still have one last bottleneck: This is the main query, generated from the report.

SELECT
    A.*,
    B.*,
    C.*,
    D.*,
    E."N",
    F."N",
    G."N"
FROM
    A
LEFT OUTER JOIN B ON
    A."PK" = B."FK"
LEFT OUTER JOIN C ON
    A."PK" = C."FK"
LEFT OUTER JOIN D ON
    A."FK" = D."PK"
LEFT OUTER JOIN E ON
    A."PK" = E."FK"
LEFT OUTER JOIN F ON
    A."PK" = F."FK"
LEFT OUTER JOIN G ON
    A."PK" = G."FK"
WHERE A.PK = ####

A,B,C and D are tables. E,F,G are simple views.

As you see, the report generated multiple LEFT JOINS. This query takes 2.28 seconds to complete (From the Plan Viewer stats). I identified three joins that seem problematic. If I remove E,F,G from the query, it becomes almost instant (0.0009s from the same stats)

SELECT
    A.*,
    B.*,
    C.*,
    D.*
FROM
    A
LEFT OUTER JOIN B ON
    A."PK" = B."FK"
LEFT OUTER JOIN C ON
    A."PK" = C."FK"
LEFT OUTER JOIN D ON
    A."FK" = D."PK"
WHERE A.PK = ####

I tought it might be the views that are slow, but if I do for example …

SELECT *
FROM E
WHERE E.FK = ####

… it is also almost instant (0.0009s)

Tables all have indexes on PKs-FKs.
Views E,F,G all return one or no row with [FK|N] as columns, so the resulting column is NULL or a number.

Do you know how I could make this query fast?

PS: If I replace LEFT OUTER JOINS by INNER JOINS the main query becomes fast… :-/

Or trying to split this query into multiple queries on the report would be a better solution?

Thank you!

Answer 1

I would create functions for the lookup against E, F and G instead of joining them.
That way there is little chance the optimiser gets confused and tries to do stupid things.

SELECT
    A.*,
    B.*,
    C.*,
    D.*,
    GET_E(A."PK"),
    GET_F(A."PK"),
    GET_G(A."PK")
FROM
    A
LEFT OUTER JOIN B ON
    A."PK" = B."FK"
LEFT OUTER JOIN C ON
    A."PK" = C."FK"
LEFT OUTER JOIN D ON
    A."FK" = D."PK"
WHERE A.PK = ####