两个链接服务器中连接两个数据库时的查询

Question

i have a query which joins Microsoft and Oracle (as Linked) but this query takes long time (more than 1 hour). but I run every joins separately, does not took long ( less than 10sec each). Is there any way I can re-do the query. eg running different part of query then getting a combined result.

SELECT A.NAME, A.NMR, B.ADDRESS, C.TEL
FROM ALPHA A

LEFT JOIN (SELECT ADDRESS, NMR FROM OPENQUERY(ORCA, 'SELECT ADDRESS, NMR FROM tblADDRESS') B ON B.NMR = A.NMR

LEFT JOIN (SELECT TEL, NMR FROM OPENQUERY(ORCT, 'SELECT TEL, NMR FROM tblTEL') C ON C.NMR = A.NMR

I want to run separately

SELECT A.NAME, A.NMR FROM ALPHA A

then

SELECT TEL, NMR FROM OPENQUERY(ORCT, 'SELECT TEL, NMR FROM tblTEL')

then

SELECT TEL, NMR FROM OPENQUERY(ORCT, 'SELECT TEL, NMR FROM tblTEL')

then

get the result

Answer 1

I would try to load each result from the OPENQUERY's into their own temp tables and then join the temp tables to the "local" table.

SELECT 
    ADDRESS
    ,NMR 
INTO #ORCA 
FROM 
    OPENQUERY(ORCA, 'SELECT ADDRESS, NMR FROM tblADDRESS')

SELECT 
    TEL
    ,NMR
INTO #ORCT
FROM 
    OPENQUERY(ORCT, 'SELECT TEL, NMR FROM tblTEL')

SELECT 
    A.NAME
    ,A.NMR
    ,ORCA.ADDRESS
    ,ORCT.TEL
FROM
    ALPHA A

        LEFT JOIN #ORCA ORCA
        ON A.NMR = ORCA.NMR

        LEFT JOIN #ORCT ORCT
        ON A.NMR = ORCT.NMR

Answer 2

I'm not sure how much of a performance improvment it will be but can try using a CTE. Part of me thinks this will do the exact same thing but if you are saying they all run faster separately maybe it will help.

with data1 as (SELECT A.NAME, A.NMR FROM ALPHA A),
data2 as(SELECT ADDRESS, NMR FROM OPENQUERY(ORCA, 'SELECT ADDRESS, NMR FROM tblADDRESS'),
data3 as(SELECT TEL, NMR FROM OPENQUERY(ORCT, 'SELECT TEL, NMR FROM tblTEL')

SELECT A.NAME, A.NMR, B.ADDRESS, C.TEL
FROM data1 A
left join data2 B ON B.NMR = A.NMR
left join data3 C ON C.NMR = A.NMR

Answer 3

I'm not sure I quite follow the question, but it's true that cross-server joins tend to be very inefficient. Probably your best approach would be to SELECT the subset of data you need from the remote server into a local temporary table, then JOIN to the temporary table.

select * into #remoteAddress from openquery (orca, 'select id, address, nmr from tblAddress');
select * into #remoteTelephone from openquery (orcb, 'select tel, nmr from tblTel');
select a.name, a.nmr, b.address, c.tel
 from alpha a
 left join #remoteAddress b
  on a.nmr = b.nmr
 left join #remoteTelephone c
  on a.nmr = c.nmr

The main point is to avoid running the JOIN across servers. Get the data you need into temporary tables and join to them instead.