slow performance with exists case statement - sql

Question

Essentially I am trying to see if c_DSS_PG_Submission.PtNum is in the ED_MLP_ATTN temp table and if it is then assign 'MLP+ATTN'. The temp table alone takes about 2 minutes to generate and has ~1000 rows, the PG table has about ~300 rows so these are not big tables. However the query below runs for 20+ minutes. Would you recommend anything different with the query? I've tried changing exists to IN but same slow performance.
WITH ed_mlp_attn
AS ( SELECT smsdss.c_cfvhs_emstat_chart.visitno ,
CAST (smsdss.c_cfvhs_emstat_chart.dschdate AS DATE) AS dschdate
FROM smsdss.c_cfvhs_emstat_chart
INNER JOIN smsdss.c_cfvhs_emstat_oi_header ON smsdss.c_cfvhs_emstat_chart.chrtno = c_cfvhs_emstat_oi_header.chartno
COLLATE SQL_Latin1_General_Pref_CP1_CI_AS
INNER JOIN smsdss.c_cfvhs_emstat_oi_detail ON c_cfvhs_emstat_oi_header.oi_header_id = smsdss.c_cfvhs_emstat_oi_detail.oi_header_id
INNER JOIN smsdss.c_cfvhs_emstat_physician ON smsdss.c_cfvhs_emstat_chart.erphys = smsdss.c_cfvhs_emstat_physician.physid
COLLATE SQL_Latin1_General_Pref_CP1_CI_AS
WHERE smsdss.c_cfvhs_emstat_chart.dschdate >= DATEADD(mm, -1,
GETDATE())
AND smsdss.c_cfvhs_emstat_chart.dispocd <> 'DXERR'
AND c_cfvhs_emstat_oi_detail.VALUE IN ( '21504',
'21505' )
AND smsdss.c_cfvhs_emstat_physician.code1 = 'RES'
)
SELECT atndrname ,
atndrno ,
CASE WHEN EXISTS ( SELECT 1
FROM ed_mlp_attn
WHERE visitno COLLATE SQL_Latin1_General_Pref_CP1_CI_AS = smsdss.c_dss_pg_submission.ptnum )
THEN 'MLP+ATTN'
ELSE 'NO'
END AS ed_prov_type
FROM smsdss.c_dss_pg_submission
WHERE date_run = '2014-12-12'
AND surveydesignator IN ( 'ER0101', 'PE0101' )
ORDER BY surveydesignator ,
ptnum

Answer 1

Firstly, a CTE is not the same as a temp table, note the information in #JodyT's comment.
The query in the CTE will be executed for each row returned by outer query.
This will slow down the query a great deal I'd expect. I would break down the current CTE in to an actual temp table as a starting point to improve performance.
NOTE: I've used aliases for table names to reduce the amount of SQL and make it a little easier to read.
SELECT chart.visitno , CAST (chart.dschdate AS DATE) AS dschdate
INTO #TEMP
FROM c_cfvhs_emstat_chart chart
INNER JOIN c_cfvhs_emstat_oi_header header
ON chart.chrtno = header.chartno COLLATE SQL_Latin1_General_Pref_CP1_CI_AS
INNER JOIN c_cfvhs_emstat_oi_detail detail
ON header.oi_header_id = detail.oi_header_id
INNER JOIN c_cfvhs_emstat_physician physician
ON chart.erphys = physician.physid COLLATE SQL_Latin1_General_Pref_CP1_CI_AS
WHERE chart.dschdate >= DATEADD(mm, -1, GETDATE())
AND chart.dispocd <> 'DXERR'
AND detail.VALUE IN ( '21504', '21505' )
AND physician.code1 = 'RES'
Then query that:
SELECT atndrname ,
atndrno ,
CASE WHEN EXISTS ( SELECT 1
FROM #TEMP
WHERE visitno COLLATE SQL_Latin1_General_Pref_CP1_CI_AS = smsdss.c_dss_pg_submission.ptnum )
THEN 'MLP+ATTN'
ELSE 'NO'
END AS ed_prov_type
FROM smsdss.c_dss_pg_submission
WHERE date_run = '2014-12-12'
AND surveydesignator IN ( 'ER0101', 'PE0101' )
ORDER BY surveydesignator , ptnum
Breaking it down like this should improve performance by a degree, but without information on indexes and an execution plan, it's difficult to provide further advice.