Optimise case/decode query in Postgres - postgresql

Question

Can anyone help me to implement the below query in another way? I am facing a performance issue because the same multiple SELECT statements are used everywhere in multiple CASE statements.
My code is very big, a chunk of it is here:
select col1,col2,
( CASE
WHEN NOT EXISTS (SELECT
CASE over.ola
WHEN NULL::text THEN 'NT'::character varying
END AS tover
FROM tsmt over
WHERE over.cod = hfprc.cod
AND over.vdat = nav.vdat) THEN 'NT'
ELSE (SELECT CASE over.ola
WHEN NULL::text THEN 'TT'::character varying END
FROM tsmt over
WHERE over.cod = hfprc.cod
AND over.vdat = nav.vdat)
END )
olaflag,
(SELECT description
FROM tolahfp tover
WHERE tover.ola = (SELECT CASE
WHEN NOT EXISTS (SELECT
CASE over.ola
WHEN NULL::text THEN 'TT'::character varying
END AS
tover
FROM tsmt over
WHERE over.cod = hfprc.cod
AND over.vdat = nav.vdat) THEN 'TT'
ELSE (SELECT CASE over.ola
WHEN NULL::text THEN 'TT'::character varying
END AS tover
FROM tsmt over
WHERE over.cod = hfprc.cod
AND over.vdat = nav.vdat)
END ola
))
ola,--ola description
(CASE thfp.codcag::text
WHEN NULL::text THEN 'CCCC'::character varying
END
)codcag,
(SELECT conf.descag
FROM thfconagrhfp conf
WHERE conf.codcag = CASE thfp.codcag::text
WHEN NULL::text THEN 'CCCC'::character varying
END)
desccag,
thfp.frq,
thfp.flgcal
FROM thpr hfprc LEFT JOIN tgrpprchfp grp ON hfprc.cgrp = grp.cgrp
LEFT JOIN tnav nav ON hfprc.cod = nav.cod AND nav.flag = 'Y'
LEFT JOIN thf thfp ON hfprc.cod = thfp.cod
LEFT JOIN tsmtdatthdhfp smtdatthd ON hfprc.cod = smtdatthd.cod

Answer 1

Do you use sqlsmith to write your queries?
But, more seriously, the way forward here is to replace subselects with outer joins.
As an example:
SELECT CASE WHEN NOT EXISTS (SELECT weirdfunc(x.x1)
FROM x
WHERE x.x2 = y.y2
AND x.x3 = z.z3)
THEN 'Constant'
ELSE (SELECT otherweirdfunc(x.x1)
FROM x
WHERE x.x2 = y.y2
AND x.x3 = z.z3)
END
FROM y
LEFT JOIN z USING (t);
would be better and more efficiently be written as
SELECT coalesce(otherweirdfunc(x.x1), 'Constant')
FROM y
LEFT JOIN z USING (t)
LEFT JOIN x ON x.x2 = y.y2 AND x.x3 = z.z3;
My rule of thumb is: subselects are usually wrong anywhere except in the FROM clause.