Btree内のposting listは圧縮されているのかな？Ginのposting listみたいに
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似たような疑問を持って、返信で情報もらったけど、まだよく分かってない。。
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I just wonder what happens if multiple keys with the same value are deduplicated and then all the others except one key are deleted. If each entry is not marked as LP_DEAD, unncessary lookup for the table can happen? twitter.com/fujii_masao/st…
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READMEにこんな記述がありました。
Note that posting list tuples can only have their LP_DEAD bit set when every table TID within the posting list is known dead.
コードもぱっと見た感じも、全Itemが死んでいる場合にのみIndexTuple全体がLP_DEADになるように見えました。
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Another mitigating factor (in addition to the thing I said before about deduplication happening lazily, after LP_DEAD deletion has already failed to avoid a split) is bitmap scans. They don't know how to set LP_DEAD bits, and are the common case with low cardinality data.