5

I have some queries where I need to get more than 64 specific rows, like this example with 65 IDs. TableID is primary key, type BigInt.

SELECT * FROM TableA 
        WHERE TableID IN (260905384, 260915601, 260929877, 260939625, 260939946, 261096977, 261147037, 261152934, 261163936, 261357728, 261369122, 261376714, 261454472, 261488500, 261527284, 261584786, 261619749, 261679560, 261777653, 261786639, 261795246, 261795810, 261803724, 261821199, 261824173, 261827397, 261840197, 261848595, 261874545, 261889122, 261889355, 261929793, 261953069, 262106609, 262134069, 262134088, 262339745, 262354363, 262360015, 262571936, 262586920, 262591486, 262663776, 262703601, 262746674, 262792439, 262801544, 262826561, 262933229, 262933270, 262947539, 262958110, 263021588, 263032875, 263037208, 263039292, 263045038, 263085369, 263089147, 263091427, 263097644, 263100021, 263103339, 263104396, 263956373)

If I check the execution plan it uses the primary key, but it executes 65 times and add a Constand Scan and Nested Loops operation item. However - if I reduce the number of parameters to 64, then it executes only 1 time directly with no other operations.

I can see that with 65+ parameters the Seek Predicates only contain one element, and if the number of parameters is 64 or less the Seek Predicates contains all of the elements directly.

Is it possible to avoid MSSQL to execute as many times a parameters when number of parameters is more than 64?

On small tables the difference is not that big, but if I join the results with other tables the numbers of reads difference becomes huge.

To reproduce this with the StackOverflow2013 database, for example:

/* 63 rows: */
SELECT *
  FROM dbo.Users
  WHERE Id IN (-1,1,2,3,4,5,8,9,10,11,13,16,17,19,20,22,23,24,25,26,27,29,30,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,48,49,50,51,52,55,56,57,58,59,60,61,62,63,64,67,68,70,71,72,73,75,76,77,78);

/* 64 rows: */
SELECT *
  FROM dbo.Users
  WHERE Id IN (-1,1,2,3,4,5,8,9,10,11,13,16,17,19,20,22,23,24,25,26,27,29,30,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,48,49,50,51,52,55,56,57,58,59,60,61,62,63,64,67,68,70,71,72,73,75,76,77,78,79);

/* 65 rows: */
SELECT *
  FROM dbo.Users
  WHERE Id IN (-1,1,2,3,4,5,8,9,10,11,13,16,17,19,20,22,23,24,25,26,27,29,30,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,48,49,50,51,52,55,56,57,58,59,60,61,62,63,64,67,68,70,71,72,73,75,76,77,78,79,80);
GO

The actual execution plans for the first two show just the clustered index seek, but the third adds a constant scan, and the estimated number of rows is suddenly incorrect:

Execution plan

In this example, there's no join. However, you can imagine that as you join to other tables, this incorrect estimation could cause the difference between a table scan versus an index seek + key lookups, and with a lowball estimate, an incorrect index seek + key lookup plan could be chosen for other tables.

  • Can you share a repro where this increases the number of reads? At the end of the day whether the seeks are driven by a constant scan passing in the value or the seek operator itself containing multiple different seek predicates it should still end up just doing an equality seek per distinct item in the "in" list – Martin Smith May 2 at 0:06
5

An IN clause gets "rewritten" to this form, essentially:

WHERE 
    TableID = 260905384
    OR TableID = 260915601
    OR TableID = 260929877
    ...

I have heard, and observed, that SQL Server has a hard coded limit of 64 OR predicates that it will put in a scan or seek operator. This isn't publicly documented anywhere as far as I can tell, and there isn't any way I'm aware of to change it either.

Beyond 64 OR expressions, as you mentioned, you end up with the "Constant Scan" plan, which drives multiple seeks or scans into the index (one for each literal).

Putting that many values literal values in an IN construction is generally considered a bad idea. If possible, change the way the query is written. You could, for example, insert all of those values into a temp table, and then do an INNER JOIN to that temp table table on the TableID column.

| improve this answer | |
  • 2
    As Josh mentions, using this IN construction will have more negative effects than the plan differences. If you run the query 27 times with different numbers of parameters, you'll go through compilation every time, and produce a different plan every time. Passing in the list as a TVP (or CSV & splitting it into a temp table) will yield a stable plan and help performance – AMtwo May 2 at 12:44
2

I tried with a temp table and INNER JOIN on it. That reduced number of reads in this case from above 4M to only 2K. Duration from 4500ms to 300ms.

In my specific case MSSQL ended up doing this as the Seek Predicates on the tables primary key:

Seek Keys[1]: Start: [DB].[dbo].[TableA].TableID >= Scalar Operator((260905384)); End: [DB].[dbo].[TableA].TableID <= Scalar Operator((263956373))

It would have been much better if it has done (as Martin Smith also expects in his comment) a seek per distinct item in the list. However, for some reason I don’t understand, it chooses a greater and less than approach instead.

So answer accepted (unfortunately I did the question as a guest user, so it seems impossible to mark it as «accepted» now)

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