SQL Multiple Left Joins性能很差

Question

所以，我有一个相当简单的查询，它有效但很慢。

有更好的方法来优化查询吗？我应该在多列上打破LEFT JOINS吗？如何进行多次运行并使用Union ALL将它们连接在一起？

这些表中可能有500,000个ROWS，并且索引很好。 请帮忙。查询需要永远运行

SELECT 
ep.Id,
ep.DisplayName,
ep.EmailAddress,
COALESCE(
    gs.DisplayNameMember + ' (' + gd.DisplayName + ')',
    ea.DisplayName ,
    ep.TrusteeUserAccountName,
    ep.Trustee
) AS Trustee

FROM 
dbo.ExchangePermissions ep WITH (NOLOCK)

LEFT JOIN dbo.GroupDetail gd WITH (NOLOCK) ON (ep.Trustee = gd.[Identity] OR ep.Trustee = gd.DisplayName OR ep.TrusteeUserAccountName = gd.SamAccountName)
LEFT JOIN dbo.GroupMemberShip gs WITH (NOLOCK) ON gd.name = gs.groupname
LEFT JOIN dbo.ExchangeAccount ea WITH (NOLOCK) ON (ep.Trustee = ea.[Identity] OR ep.Trustee = ea.DisplayName OR ep.TrusteeUserAccountName = ea.SamAccountName)

WHERE
ep.OTHERID= @MyParameter

Answer 1

OR中的LEFT JOIN根本没有帮助表现。您应该将它们分解为单独的查询并将UNION结果合并在一起。在Vykintas的回答中，我们得到以下结论：

SELECT 
ep.Id,
ep.DisplayName,
ep.EmailAddress,
COALESCE(
    gs.DisplayNameMember + ' (' + gd.DisplayName + ')',
    ep.TrusteeUserAccountName,
    ep.Trustee
) AS Trustee
FROM 
dbo.ExchangePermissions ep WITH (NOLOCK)
LEFT JOIN dbo.GroupDetail gd WITH (NOLOCK) ON ep.Trustee = gd.[Identity] 
LEFT JOIN dbo.GroupMemberShip gs WITH (NOLOCK) ON gd.name = gs.groupname
WHERE
ep.OTHERID= @MyParameter

UNION

SELECT 
ep.Id,
ep.DisplayName,
ep.EmailAddress,
COALESCE(
    gs.DisplayNameMember + ' (' + gd.DisplayName + ')',
    ep.TrusteeUserAccountName,
    ep.Trustee
) AS Trustee
FROM 
dbo.ExchangePermissions ep WITH (NOLOCK)
LEFT JOIN dbo.GroupDetail gd WITH (NOLOCK) ON ep.Trustee = gd.DisplayName
LEFT JOIN dbo.GroupMemberShip gs WITH (NOLOCK) ON gd.name = gs.groupname
WHERE
ep.OTHERID= @MyParameter

UNION

SELECT 
ep.Id,
ep.DisplayName,
ep.EmailAddress,
COALESCE(
    gs.DisplayNameMember + ' (' + gd.DisplayName + ')',
    ep.TrusteeUserAccountName,
    ep.Trustee
) AS Trustee
FROM 
dbo.ExchangePermissions ep WITH (NOLOCK)
LEFT JOIN dbo.GroupDetail gd WITH (NOLOCK) ON ep.TrusteeUserAccountName = gd.SamAccountName
LEFT JOIN dbo.GroupMemberShip gs WITH (NOLOCK) ON gd.name = gs.groupname
WHERE
ep.OTHERID= @MyParameter