SQL Server indexes Work Sometimes [duplicate]

Question

This question already has answers here:

Struggling to create an efficient index for this query (3 answers)

Closed 1 year ago.

So I have the following indexes on my table:

CREATE NONCLUSTERED INDEX [$5] ON [dbo].[Record Link]
(
    [Company] ASC,
    [Record ID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON, FILLFACTOR = 80) ON [PRIMARY]
GO

 CREATE NONCLUSTERED INDEX [$6] ON [dbo].[Record Link]
(
    [Notify] ASC,
    [Record ID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON, FILLFACTOR = 80) ON [PRIMARY]
GO

CREATE NONCLUSTERED INDEX [$7] ON [dbo].[Record Link]
(
    [Company] ASC,
    [Type] ASC,
    [Link ID] ASC,
    [Record ID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON, FILLFACTOR = 80) ON [PRIMARY]
GO

CREATE NONCLUSTERED INDEX [$8] ON [dbo].[Record Link]
(
    [Company] ASC,
    [Type] ASC,
    [Record ID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON, FILLFACTOR = 80) ON [PRIMARY]
GO

CREATE NONCLUSTERED INDEX [bain_test] ON [dbo].[Record Link]
(
    [Record ID] ASC,
    [Type] ASC,
    [Company] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON, FILLFACTOR = 80) ON [PRIMARY]
GO

CREATE NONCLUSTERED INDEX [$4] ON [dbo].[Record Link]
(
    [Notify] ASC,
    [Company] ASC,
    [Link ID] ASC,
    [To User ID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON, FILLFACTOR = 80) ON [PRIMARY]
GO

The Problem is that some of the indexes only get used some times. As an example look at this query and the Execution plan, you can see that it is executed within the EXISTS but it is not executed for the rest of the query. Both where clauses are identical.

Here is that queries statistics time and IO:

*SQL Server Execution Times:
   CPU time = 0 ms,  elapsed time = 0 ms.
Table 'Record Link'. Scan count 1, logical reads 4, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.*

*(1 row affected)*

*SQL Server Execution Times:
   CPU time = 0 ms,  elapsed time = 0 ms.*
*(1 row affected)*

*Table 'Record Link'. Scan count 1, logical reads 1017129, physical reads 0, read-ahead reads 0, lob logical reads 1, lob physical reads 0, lob read-ahead reads 0.*

*(1 row affected)*

*SQL Server Execution Times:
   CPU time = 1969 ms,  elapsed time = 1963 ms.*

I don't have the option of changing the query as it from an ERP system and I'm trying to optimize the speed.

And while we are here there are a few different variations for querying this table.

Here are another one that don't use the indexes at all.

With this one things are strange, so i first tried having an index that uses fields: Record ID, Company, this didn't work until I created a new index and made it: Company, Record ID.

I Did this yesterday and it was working, but this morning in the profiler its not using the index again.

SELECT  TOP (1) 
    "timestamp","Link ID",
    "Record ID",
    "URL1",
    "URL2",
    "URL3",
    "URL4",
    "Description",
    "Type",
    DATALENGTH("Note"),
    "Created",
    "User ID",
    "Company",
    "Notify",
    "To User ID" 
FROM 
    "Live".dbo."Record Link" WITH(UPDLOCK)  
WHERE 
    ("Record ID"=@0 AND "Company"=@1) 
ORDER BY 
    "Link ID" ASC 
OPTION(OPTIMIZE FOR UNKNOWN)

With this one, i really just cant figure it out, i thought the index with company,notify,User ID would work:

SELECT TOP (@0)
    "timestamp", "Link ID", "Record ID",
    "URL1", "URL2", "URL3", "URL4",
"Description", "Type", DATALENGTH("Note"), "Created", "User ID",
    "Company", "Notify", "To User ID"
FROM "Live_Replica".dbo."Record Link" WITH(READUNCOMMITTED)
WHERE ("Link ID">@1
  AND (("Company"=@2 OR "Company"=@3)
  AND "Notify"=@4
  AND ("To User ID" COLLATE Latin1_General_100_CI_AI LIKE @5
    OR "To User ID" COLLATE Latin1_General_100_CI_AI LIKE @6)))
ORDER BY "Link ID" ASC
OPTION(OPTIMIZE FOR UNKNOWN, FAST 50)

I Have tried googling for this issues but cant seem to find an answer at all. All indexes are rebuilt every night and i don't shrink the database at all so that the indexes don't get fragmented.

Answer 1

As an example look at this query and the Execution plan, you can see that it is executed within the EXISTS but it is not executed for the rest of the query. Both where clauses are identical.

Both where clauses are identical, but the select lists are not.

There is NO covering index for your queries among those shown in your question, except for those with exists.

For exists query it's important only to know if such a record (with [Record ID] = ..., [Type] = ...,[Company] = ...) exists, so the server can only control your index [bain_test] that includes all these fields as key fiels.

For the second query you asked server for many other fields (see your SELECT list) that are not included in this index and in no other index, so if the server chose your index it had to do lookups to a base table, and since statistics cannot be used in your case (you used oprimize for unknown hint), server estimated the number of returned rows as too many and chosed to do a scan. Scan at least is continuos reading while lookups are random and they cost.

The same is for your third query. None of the indexes is covering, you select the fields that are not included in your indexes, and even if it seems that this query should return only one row, it's not a random row, your top has order by clause, so all the records should be read first and ten sorted.

I think you should examing your missimg indexes from

sys.dm_db_missing_index_groups, sys.dm_db_missing_index_group_stats,

sys.dm_db_missing_index_details

to decide what indexes you should create.

And also take a look on sys.dm_db_index_usage_stats to find out unused indexes.

Your indexes are very similar, and your conditions are similar too, at least all the queryis have Record ID AND Company. Why don't you think that your clustered index (on [Record ID] I suppose) is not optimal? It's already sorted on one of your conditions, it is very probably unique and it's always covering. It's very probably that your indexes are not used at all.

And please update your quastion with clustered index definition.