Hello,
Thank you for your comments.
Following Corey's advice, I applied Amit's patches proposed in this
email [1], and confirmed our memory pressure problem was solved.
So dropping cached plan with DISCARD is not necessary anymore.
[1] https://www.postgresql.org/message-id/CA%2BHiwqG1qQuBwApueaUfA855UJ4TiSgFkPF34hQDWx3tOChV5w%40mail.gmail.com
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Best regards,
Yuzuko Hosoya
NTT Open Source Software Center