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Need some help in understanding the slowness in one of the UPDATE statements like below:-

UPDATE TOP (100) xyz
SET xyz.flag = 1
OUTPUT inserted.Rcode, inserted.EDR, inserted.id, abc.EID,abc.CID,abc.ENID,abc.Cdate
FROM dbo.table1 xyz WITH (UPDLOCK, READPAST)
INNER JOIN dbo.table2 abc WITH (NOLOCK)
on xyz.id=abc.id
WHERE xyz.flag = 0

Table1 has approx. 0.5 million rows and Table 2 has approx. 5 million rows

Slow Plan

Hash Match distinct flow operator shows a yellow alarm and message is:

Operator used Tempdb to spill data for executed with spill level 4 with 1 spilled thread"

Build residual:

database.dbo.table2.id as abc.id = database.dbo.table2.id as abc.id

I took a screenshot. Unfortunately due to security reasons I can't provide more than that, not even an anonymized plan. From my working station I cannot access the internet so there is no way I can get plan explorer to run there.

enter image description here

Generally for a smaller subset of rows it's under sec like when we have just matching 10K rows or something. But with higher amount of data this seems to be tipping point and app cannot afford 1 min of run time. From SSMS I get 30 secs but from app we have avg. 50 secs approx. RCSI is in testing phase.

My good plan does not have that Hash Match Flow Distinct operator visible as shown in my screenshot, while rest of plan remains same. Good one completes under 3 secs or so. As seen nearly 16 seconds are spent on that operator. Can we eliminate it via proper indexing or query re-write?

Table schema

CREATE TABLE dbo.table1
(
    Recid  VARCHAR(128) COLLATE SQL_Latin1_general_CP1_CI_AS NOT NULL,
    Cdate DATETIME NULL,
    flag BIT   NULL DEFAULT (0),
    Rcode INT NULL,
    EDR VARCHAR(255) COLLATE SQL_Latin1_general_CP1_CI_AS NULL,
    id BIGINT NULL
);

CREATE TABLE dbo.table2
(
    ENID BIGINT IDENTITY(1,1) NOT NULL,
    EID VARCHAR(50) COLLATE SQL_Latin1_general_CP1_CI_AS NOT NULL,
    CID VARCHAR(350) COLLATE SQL_Latin1_general_CP1_CI_AS NOT NULL,
    CDate DATETIME NOT NULL DEFAULT(getdate()),
    id BIGINT NOT NULL,

    CONSTRAINT PK_ENID PRIMARY KEY (ENID ASC, EID ASC),
);
-- table1
CREATE INDEX ix_Cdate on dbo.table1 (Cdate) WITH (FILLFACTOR=100);
CREATE CLUSTERED INDEX ix_Recid on dbo.table1 (Recid) WITH (FILLFACTOR=80);

-- table2
CREATE INDEX ix_ENID_id on dbo.table2 (ENID,id) WITH (FILLFACTOR=100);

Changes

Changes I made and some numbers:

  • Added hint OPTION (QUERYTRACEON 4138) - avg. execution 7 secs down from original 50secs, but app team seems not have access to perform this in code. Need to check further on this.

  • OPTION (ORDER GROUP)gave same results of avg. 50secs so no improvement there.

  • Added index as suggested:

CREATE INDEX i ON dbo.table2 (id) INCLUDE (CID, CDate);

Not much improvements there. Avg. 45 secs and plan was similar to one attached in this question (top plan).

Before and after each test I made sure plan was not generated from previous cached plan.

Fast plan

Attaching the plan which is faster and without any change in data or query is still fast for same amount of rows in both tables. App team continuously submit above query throughout the day to finish the batch by completing those TOP 100. There is a plan change based on some tipping number and below is how the good plan looks:

enter image description here

Edit:- With everything unchanged, no code change or any index being added, as suggested when i am trying to add hint (FORCESEEK) its giving me below error

Query processor could not produce a query plan because of the hints defined in this query. Resubmit the query without specifying any hints and without using SET FORCEPLAN.

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1 Answer 1

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You have three main problems:

  1. There is no useful index to support the join on id.
  2. The TOP (100) introduces a row goal, so estimations may be too low.
  3. The UPDATE is non-deterministic.

Multiple rows from table2 could match on id, so it is not clear which matching row from table2 should be used to provide values for the OUTPUT clause. The aggregate is there to group on table2 id and choose ANY matching values for the other columns. The aggregate is a Flow Distinct because of the row goal.

One needs to be very careful with ANY aggregates in non-deterministic UPDATE statements because you may get incorrect results.

There is not enough detail in the question to make high quality recommendations, but:

  1. Add an index like CREATE INDEX i ON dbo.table2 (id) INCLUDE (CID, CDate);
  2. Use OPTION (QUERYTRACEON 4138) to disable the row goal, or OPTION (ORDER GROUP) to use a Stream Aggregate instead of Hash.
  3. How you fix the non-deterministic UPDATE depends on the data relationships. The key point is to identify at most one row from the source that matches each target row. Typically, this will involve a unique index or constraint, or using ROW_NUMBER or TOP (1).

Step 2 may or may not be necessary. I add it for completeness.

You may find it easier to visualize the issues and tune the query by writing it in this form:

UPDATE TOP (100) 
    xyz WITH (UPDLOCK, READPAST)
SET xyz.flag = 1
OUTPUT 
    inserted.Rcode, inserted.EDR, inserted.id, 
    abc.EID, abc.CID, abc.ENID, abc.Cdate
FROM dbo.table1 AS xyz
CROSS APPLY
(
    -- At most one source row per target row
    SELECT TOP (1) 
        abc.* 
    FROM dbo.table2 AS abc
    WHERE
        abc.id = xyz.id
    -- ORDER BY something to choose the one row
) AS abc
WHERE 
    xyz.flag = 0;

Execution plan:

Expected plan shape

I probably wouldn't bother with a filtered index on table1, but if you did want to try it, this appears to be suitable:

CREATE INDEX i 
ON dbo.table1 (Recid) 
INCLUDE (id, flag) 
WHERE flag = 0;

If you want to continue with the update syntax given in the question without addressing all the underlying issues properly, you may find this is faster:

UPDATE TOP (100) xyz
SET xyz.flag = 1
OUTPUT inserted.Rcode, inserted.EDR, inserted.id, abc.EID,abc.CID,abc.ENID,abc.Cdate
FROM dbo.table1 xyz WITH (UPDLOCK, READPAST)
INNER JOIN dbo.table2 abc WITH (NOLOCK, FORCESEEK)
on xyz.id=abc.id
WHERE xyz.flag = 0;
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  • @ Thanks Paul for all your inputs. No other option worked than to change code for CROSS APPLY. We did it however one question which was asked by mydevs which they think ORDER BY is not needed. In your query you mention to sort by one of the column?
    – Newbie-DBA
    Aug 11, 2020 at 20:32
  • We deployed the code in prod with cross apply and somehow it started to go even more slow now. table A has approx 300K rows while table B is approx 5 M rows. Any help is appreciated. The exec plan is similar to one you have in your answer
    – Newbie-DBA
    Sep 10, 2020 at 13:22
  • Changes we did was add index and use cross apply for top 1
    – Newbie-DBA
    Sep 10, 2020 at 13:24

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