optimize Table Spool in SQL Server Execution plan - sql

Question

I have the following sql query and trying to optimize it using execution plan. In execution plan it says Estimated subtree cost is 36.89. There are several table spools(Eager Spool). can anyone help me to optimize this query. Thanks in advance.
SELECT
COUNT(DISTINCT bp.P_ID) AS total,
COUNT(DISTINCT CASE WHEN bc.Description != 'S' THEN bp.P_ID END) AS m_count,
COUNT(DISTINCT CASE WHEN bc.Description = 'S' THEN bp.P_ID END) AS s_count,
COUNT(DISTINCT CASE WHEN bc.Description IS NULL THEN bp.P_ID END) AS n_count
FROM
progress_tbl AS progress
INNER JOIN Person_tbl AS bp ON bp.P_ID = progress.person_id
LEFT OUTER JOIN Status_tbl AS bm ON bm.MS_ID = bp.MembershipStatusID
LEFT OUTER JOIN Membership_tbl AS m ON m.M_ID = bp.CurrentMembershipID
LEFT OUTER JOIN Category_tbl AS bc ON bc.MC_ID = m.MembershipCategoryID
WHERE
logged_when BETWEEN '2017-01-01' AND '2017-01-31'

Answer 1

Here's a technique you can use.
WITH T AS
(
SELECT DISTINCT CASE
WHEN bc.Description != 'S' THEN 'M'
WHEN bc.Description = 'S' THEN 'S'
WHEN bc.Description IS NULL THEN 'N'
END AS type,
bp.P_ID
FROM progress_tbl AS progress
INNER JOIN Person_tbl AS bp
ON bp.P_ID = progress.person_id
LEFT OUTER JOIN Status_tbl AS bm
ON bm.MS_ID = bp.MembershipStatusID
LEFT OUTER JOIN Membership_tbl AS m
ON m.M_ID = bp.CurrentMembershipID
LEFT OUTER JOIN Category_tbl AS bc
ON bc.MC_ID = m.MembershipCategoryID
WHERE logged_when BETWEEN '2017-01-01' AND '2017-01-31'
)
SELECT COUNT(DISTINCT P_ID) AS total,
COUNT(CASE WHEN type= 'M' THEN P_ID END) AS m_count,
COUNT(CASE WHEN type= 'S' THEN P_ID END) AS s_count,
COUNT(CASE WHEN type= 'N' THEN P_ID END) AS n_count
FROM T
I will demonstrate it on a simpler example.
Suppose your existing query is
SELECT
COUNT(DISTINCT number) AS total,
COUNT(DISTINCT CASE WHEN name != 'S' THEN number END) AS m_count,
COUNT(DISTINCT CASE WHEN name = 'S' THEN number END) AS s_count,
COUNT(DISTINCT CASE WHEN name IS NULL THEN number END) AS n_count
FROM master..spt_values;
You can rewrite it as follows
WITH T AS
(
SELECT DISTINCT CASE
WHEN name != 'S'
THEN 'M'
WHEN name = 'S'
THEN 'S'
ELSE 'N'
END AS type,
number
FROM master..spt_values
)
SELECT COUNT(DISTINCT number) AS total,
COUNT(CASE WHEN type= 'M' THEN number END) AS m_count,
COUNT(CASE WHEN type= 'S' THEN number END) AS s_count,
COUNT(CASE WHEN type= 'N' THEN number END) AS n_count
FROM T
Note the rewrite is costed as considerably cheaper and the plan is much simpler.

Answer 2

As already pointed out, there seems to be some typo/copy paste issues with your query. This makes it rather difficult for us to figure out what's going on.
The table-spools probably are what's going on in the CASE WHEN b.description etc... constructions. MSSQL first creates a (memory) table with all the resulting values and then that one gets sorted and streamed through the COUNT(DISTINCT ...) operator. I don't think there is much you can do about that as the work needs to be done somewhere.
Anyway, some remarks and wild guesses:
I'm guessing that logged_when is in the progress_tbl table?
If so, do you really need to LEFT OUTER JOIN all the other tables? From what I can tell they aren't being used?
You're trying to count the number of P_IDs that match the criteria and you want to split up that number between those that have b.Description either 'S', something else, or NULL.
for this you could calculate the total as the sum of the m_count, s_count and n_count. This would save you 1 COUNT() operation, not sure it helps a lot in the bigger picture but all bits help I guess.
Something like this:
;WITH counts AS (
SELECT
COUNT(DISTINCT CASE WHEN b.Description != 'S' THEN b_p.P_ID END) AS m_count,
COUNT(DISTINCT CASE WHEN b.Description = 'S' THEN b_p.P_ID END) AS s_count,
COUNT(DISTINCT CASE WHEN b.Description IS NULL THEN b_p.P_ID END) AS n_count
FROM
progress_tbl AS progress
INNER JOIN Person_tbl AS bp ON bp.P_ID = progress.person_id
LEFT OUTER JOIN Status_tbl AS bm ON bm.MS_ID = bp.MembershipStatusID -- really needed?
LEFT OUTER JOIN Membership_tbl AS m ON m.M_ID = bp.CurrentMembershipID -- really needed?
LEFT OUTER JOIN Category_tbl AS bc ON bc.MC_ID = m.MembershipCategoryID -- really needed?
WHERE
logged_when BETWEEN '2017-01-01' AND '2017-01-31' -- what table does logged_when column come from????
)
SELECT total = m_count + s_count + n_count,
*
FROM counts
UPDATE
BEWARE: Using the answer/example code of Martin Smith I came to realize that total isn't necessarily the sum of the other fields. It could be a given P_ID shows up with different description which then might fall into different categories. Depending on your data it might thus be that my answer is plain wrong.