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Ask Question Asked 1 year, 9 months ago. Parallel bitmap heap scan × First at 2016-10-07 06:16:40 by Dilip Kumar
Learning Bitmap Index Scan, Recheck Cond, and Bitmap Heap Scan As Postgres scans an index and finds a matching key, it can choose not to read the matching row from the table right away. (12 replies) I'm encountering an issue where PG 9.2.4 (we also see this with 9.2.3) is picking a plan involving a bitmap heap scan that turns out to be much slower than a nested-loop plan using indexes. (So both of yours rows returned were in the same block) This agrees with the Buffers data. はじめに PostgreSQL の実行計画において、Bitmap Index Scan の後に実行される Bitmap Heap Scan で "Recheck cond" と出力されます。Index Scan をしているにも関わらず、なぜ Heap Scan でインデックスの検索条件を再チェックする必要があるのか解説します。事前に以下の資料を一読頂くことをお勧めし … Learn more PostgreSQL Bitmap Heap Scan on index is very slow but Index Only Scan is fast
The Bitmap Index Scans and BitmapAnd in total hit 7 blocks. The planner thinks the bitmap scan is faster. The bitmap is a set of physical locations to scan, and the Bitmap Heap Scan will scan them in sorted order of physical location. Plus each page referred to by the bitmap will only be visited once in the scan. The Heap Scan hit one more, bringing it up to 8. A Postgresql Bitmap Heap Scan won't revert to random access. "Heap Blocks: exact=1" means that it only visited one block, and that it was not lossy.
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