If at the sort step of the first query you expect to return 2178 rows,
why would you expect to return 1 row after self-joining those results
back on to the original table?
rms=# explain analyze
rms-# select * from
rms-# ( SELECT
rms(# host_id, max(mtime) as mtime
rms(# FROM
rms(# software_download
rms(# WHERE
rms(# bds_status_id not in (6,17,18)
rms(# GROUP BY
rms(# host_id, software_binary_id
rms(# ) latest_download
rms-# JOIN software_download using (host_id,mtime);
QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------------------------------
Merge Join (cost=646.29..1346.89 rows=1 width=97) (actual
time=126.815..225.881 rows=10870 loops=1)
Merge Cond: ("outer".mtime = "inner".mtime)
Join Filter: ("inner".host_id = "outer".host_id)
-> Index Scan using sd_rht_1_idx on software_download (cost=0.00..628.95
rows=13416 width=97) (actual time=0.018..28.508 rows=13416 loops=1)
-> Sort (cost=646.29..651.74 rows=2178 width=12) (actual
time=126.775..139.811 rows=10870 loops=1)
Sort Key: latest_download.mtime
-> Subquery Scan latest_download (cost=476.53..525.54 rows=2178
width=12) (actual time=49.643..95.524 rows=10870 loops=1)
-> HashAggregate (cost=476.53..503.76 rows=2178 width=16)
(actual time=49.639..69.748 rows=10870 loops=1)
-> Seq Scan on software_download (cost=0.00..377.78
rows=13167 width=16) (actual time=0.006..24.266 rows=13167 loops=1)
Filter: ((bds_status_id <> 6) AND (bds_status_id <>
17) AND (bds_status_id <> 18))
Total runtime: 239.806 ms
(11 rows)
Robert Treat
--
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