I found this in https://wiki.postgresql.org/wiki/Todo :
Improve ability to display optimizer analysis using OPTIMIZER_DEBUG
What does this actually mean?
Add GUC switch to enable optimizer debug on/off?
More fancy/useful info should be printed?
If so, what kind of information is required?
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Tatsuo Ishii
SRA OSS, Inc. Japan
English: http://www.sraoss.co.jp/index_en.php
Japanese: http://www.sraoss.co.jp