Bitmap scans
If you only select a few rows, Postgre will decide on an index scan – if you select a majority of them, Postgre will decide on a table scan. But what if you read too much for an index scan to be efficient but too little for a sequential scan? The solution to the problem is to use a bitmap scan. This way, a single block is only used once during a scan. Postgre will first scan the index and compile those rows / blocks, which are needed at the end of the scan. Then Postgre will take this list and go to the table to really fetch those rows.
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