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 -- Best regards, Yuzuko Hosoya NTT Open Source Software Center
