Speed up SQL query performance with nested queries - sql

Question

Could anyone help me speed this query up? It currently take 17 minutes to run but does return the correct data and it populates a subform in MS Access. Functions in the rest of the VBA are declared as long to try to speed up more.
Here's the full query:
SELECT lots of things
FROM (((((((((((((((ngstest
INNER JOIN patients
ON ngstest.internalpatientid = patients.internalpatientid)
INNER JOIN referral
ON ngstest.referralid = referral.referralid)
INNER JOIN checker
ON ngstest.bookby = checker.check1id)
INNER JOIN ngspanel
ON ngstest.ngspanelid = ngspanel.ngspanelid)
LEFT JOIN ngspanel AS ngspanel_1
ON ngstest.ngspanelid_b = ngspanel_1.ngspanelid)
INNER JOIN status
ON ngstest.statusid = status.statusid)
INNER JOIN dbo_patient_table
ON patients.patientid = dbo_patient_table.patienttrustid)
LEFT JOIN dna
ON ngstest.dna = dna.dnanumber)
INNER JOIN status AS status_1
ON patients.s_statusoverall = status_1.statusid)
LEFT JOIN gw_gendertable
ON dbo_patient_table.genderid = gw_gendertable.genderid)
LEFT JOIN ngswesbatch
ON ngstest.wesbatch = ngswesbatch.ngswesbatchid)
LEFT JOIN checker AS checker_1
ON ngstest.check1id = checker_1.check1id)
LEFT JOIN checker AS checker_2
ON ngstest.check2id = checker_2.check1id)
LEFT JOIN checker AS checker_3
ON ngstest.check3id = checker_3.check1id)
LEFT JOIN ngspanel AS ngspanel_2
ON ngstest.ngspanelid_c = ngspanel_2.ngspanelid)
LEFT JOIN checker AS checker_4
ON ngstest.check4id = checker_4.check1id
WHERE ((ngstest.referralid IN
(SELECT referralid FROM referral
WHERE grouptypeid = 14)
AND ngstest.ngstestid IN
(SELECT ngstest.ngstestid
FROM ngsanalysis
INNER JOIN ngstest
ON ngsanalysis.ngstestid = ngstest.ngstestid
WHERE ngsanalysis.pedigree = 3302) )
AND status.statusid = 1202218800)
ORDER BY ngstest.priority,
ngstest.daterequested;
The two nested queries are strings from elsewhere in the code so are called in the vba as " & includereferralls & " And " & ParentsStatusesFilter & "
They are:
ParentsStatusesFilter = "NGSTest.NGSTestID in
(SELECT NGSTest.NGSTestID
FROM NGSAnalysis
INNER JOIN NGSTest
ON NGSAnalysis.NGSTestID = NGSTest.NGSTestID
WHERE NGSAnalysis.Pedigree IN (3302,3303,3304)"
And
includereferrals = "NGSTest.ReferralID
(SELECT referralid FROM referral WHERE referral.grouptypeid = 14)"
The query needs to remain readable (and therefore editable) so can't use things like Distinct, Group By or contain any Unions. Have tried Exists instead of In for the nested queries but that stops it from actually filtering the results.
WHERE EXISTS (SELECT NGSTest.NGSTestID
FROM NGSAnalysis
INNER JOIN NGSTest
ON NGSAnalysis.NGSTestID = NGSTest.NGSTestID
WHERE NGSAnalysis.Pedigree IN (3302,3303,3304)

Answer 1

So the exist clause you have there isn't tied to the outer query which would run similar to just added 1 = 1 to the where clause. I took your where clause and converted it. It should look something like this...
WHERE EXISTS (
SELECT referralid
FROM referral
WHERE grouptypeid = 14 AND ngstest.referralid = referral.referralid)
AND EXISTS (
SELECT ngsanalysis.ngstestid
FROM ngsanalysis
WHERE ngsanalysis.pedigree IN (3302,3303,3304) AND ngstest.ngstestid = ngsanalysis.ngstestid
)
AND status.statusid = 1202218800
Adding exists will speed it up a bit, but the the bulk of the slowness is the left joins. Access does not handle the left joins as well as SQL Server does. Change all your joins to inner joins and you will see the query runs very fast. This is obviously not ideal since some relationships are optional. What I have done to get around this is add a default record that replaces a null relationship.
Here is what that looks like for you: In the checker table you could add a record that represents a null value. So put a record into the checker table with check1id of -1 or 0. Then default check1id, check2id, check3id on ngstest to -1 or 0. You will need to do that type of thing for all tables you need to left join on.