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但是，可以同时使用这些索引吗? PostgreSQL为多个索引提供bitmapAnd及bitmapOr接口。它们可以组合多个索引，减少需要扫描的数据库数量。
- Heap, one square = one page:
- +---------------------------------------------+
- |c____u_____X___u___X_________u___cXcc______u_|
- +---------------------------------------------+
- Rows marked c match customers pkey condition.
- Rows marked u match username condition.
- Rows marked X match both conditions.
- Bitmap scan from customers_pkey:
- +---------------------------------------------+
- |100000000001000000010000000000000111100000000| bitmap 1
- +---------------------------------------------+
- One bit per heap page, in the same order as the heap
- Bits 1 when condition matches, 0 if not
- Bitmap scan from ix_cust_username:
- +---------------------------------------------+
- |000001000001000100010000000001000010000000010| bitmap 2
- +---------------------------------------------+
- Once the bitmaps are created a bitwise AND is performed on them:
- +---------------------------------------------+
- |100000000001000000010000000000000111100000000| bitmap 1
- |000001000001000100010000000001000010000000010| bitmap 2
- &&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&
- |000000000001000000010000000000000010000000000| Combined bitmap
- +-----------+-------+--------------+----------+
- | | |
- v v v
- Used to scan the heap only for matching pages:
- +---------------------------------------------+
- |___________X_______X______________X__________|
- +---------------------------------------------+
- The bitmap heap scan then seeks to the start of each page and reads the page:
- +---------------------------------------------+
- |___________X_______X______________X__________|
- +---------------------------------------------+
- seek------->^seek-->^seek--------->^
- | | |
- ------------------------
- only these pages read
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