引數
預設值說明
enable_bitmapscan
on啟用或禁用位圖索引掃瞄
enable_gathermerge
on啟用或禁用收集合併，併發查詢時會出現收集合併
enable_hashagg
on啟用或禁用hashaggregate
enable_hashjoin
on啟用或禁用hash join
enable_indexscan
on啟用或禁用索引掃瞄
enable_indexonlyscan
on啟用或禁用index only scan
enable_material
on啟用或禁用物化中間結果，不建議關閉
enable_mergejoin
on啟用或禁用merge join
enable_nestloop
on啟用或禁用nested loop
enable_seqscan
on啟用或禁用全表掃瞄
enable_sort
on啟用或禁用顯示排序
enable_tidscan
on啟用或禁用行id掃瞄
預設analyze會不定期自動執行。但在大量資料修改後，可能需要手動analyze生成新的統計值
當查詢只涉及到2-3張表聯表時，規劃器可以生成較優的表連線順序。規劃器分析時間隨著表的增多指數級增加。如果表太多，規劃器將使用遺傳概率搜尋，但不一定能找到最佳表連線順序。
因此需要通過join指定表連線順序
只有full join完全指定了連線順序
另外外連線優先順序低於內連線
select * from a left join (b join c on (b.ref = c.id)) on (a.id = b.id);可通過調整postgres執行時引數改變規劃器行為
如果要強制指定規劃器使用sql的連線順序，可以將from_collapse_limit，join_collapse_limit 設定為1，防止規劃器重新排序
預設情況下
explain select * from inv_t_invoice i join inv_t_invoice_detail d on i.cl_id = d.cl_invoice_id
join inv_t_invoice_cash c on i.cl_id = c.cl_invoice_id;
#result
hash join (cost=531.63..8435.58 rows=15579 width=3086)
hash cond: ((i.cl_id)::text = (d.cl_invoice_id)::text)
-> nested loop (cost=0.29..7545.20 rows=14881 width=2860)
-> seq scan on inv_t_invoice i (cost=0.00..2278.72 rows=8972 width=2385)
-> index scan using idx_invoice_cash_invoice_id on inv_t_invoice_cash c (cost=0.29..0.57 rows=2 width=475)
index cond: ((cl_invoice_id)::text = (i.cl_id)::text)
-> hash (cost=413.93..413.93 rows=9393 width=226)
-> seq scan on inv_t_invoice_detail d (cost=0.00..413.93 rows=9393 width=226)
set join_collapse_limit=2;
explain select * from inv_t_invoice i join inv_t_invoice_detail d on i.cl_id = d.cl_invoice_id
join inv_t_invoice_cash c on i.cl_id = c.cl_invoice_id;
nested loop (cost=531.63..8483.11 rows=15579 width=3086)
join filter: ((i.cl_id)::text = (c.cl_invoice_id)::text)
-> hash join (cost=531.34..3026.53 rows=9393 width=2611)
hash cond: ((i.cl_id)::text = (d.cl_invoice_id)::text)
-> seq scan on inv_t_invoice i (cost=0.00..2278.72 rows=8972 width=2385)
-> hash (cost=413.93..413.93 rows=9393 width=226)
-> seq scan on inv_t_invoice_detail d (cost=0.00..413.93 rows=9393 width=226)
-> index scan using idx_invoice_cash_invoice_id on inv_t_invoice_cash c (cost=0.29..0.56 rows=2 width=475)
index cond: ((cl_invoice_id)::text = (d.cl_invoice_id)::text)
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