[SQL] 2 Selects 1 is faster, why?

[Eric]

If I perform the following 2 selects, the first one is EXTREMELY slow where
the 2nd one is very fast.

(1) Slow

select
  o.orderid,
  ol.itemcode,
  ol.itemname

from
  orders o,
  orlines ol

where
  o.orderid = '1234' and
  ol.orderid = o.orderid;

(2) VERY FAST

select
  o.orderid,
  ol.itemcode,
  ol.itemname

from
  orders o,
  orlines ol

where
  o.orderid = '1234' and
  ol.orderid = '1234'

Why would 2 be so much faster?  I have ran the EXPLAIN on this and index
scans are being used.

NOTE: The actual queries return more information than this, but the
fundamental change shown above seems to give me the instant response I am
looking for. (1) takes about 60 seconds to run and (2) takes 3-5 seconds to
run.

Thanks, Eric





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