Firebird hash join
阿新 • • 發佈:2018-07-14
detail express sql access 哈希 fire alt lan can
Firebird 現可支持哈希連接(hash join),各中大型數據庫,哈希連接已成為平常,相對於循環嵌套連接(Nested Loop Join),在數據量較大的情況下,哈希連接性能較好。
由於 Firebird 的sql查詢優化器做的不是很好,需要使用者顯式的指定是否使用hash join,否則默認就用循環嵌套連接。
在sql語句上可以這樣標示,來使用hash join:
1 select * from b_inbound_puton a 2 join b_inbound_detail b on a.inbound_dt_id +0 = b.id +0 3 join b_inbound c onb.inbound_id +0 = c.id +0 4 where c.id = 0
即,在連接的字段上用 +0 來指定hash join。
執行計劃:
1 Select Expression 2 -> Filter 3 -> Hash Join (inner) 4 -> Filter 5 -> Hash Join (inner) 6 -> Filter 7 -> Table "B_INBOUND" as"C" Access By ID 8 -> Bitmap 9 -> Index "PK_B_INBOUND" Unique Scan 10 -> Record Buffer (record length: 321) 11 -> Table "B_INBOUND_DETAIL" as "B" Full Scan 12 -> Record Buffer (record length: 313) 13 -> Table "B_INBOUND_PUTON" as "A" Full Scan
PLAN HASH (HASH (C INDEX (PK_B_INBOUND), B NATURAL), A NATURAL)
Firebird hash join