Hi:
I've a question regarding nested loops and the order in which they return rows. Can you help me, please?
Suppose that I've two tables:
- Table [sales_order]
* Columns [id]
* Index [sales_order_pkey] on [id]
- Table [order_line]
* Columns [id], [sales_order_id]
* Index [order_line_ukey] on [sales_order_id], [id]
Then, I run the following query:
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SELECT sales_order.id, order_line.id
FROM main.sales_order
JOIN main.order_line ON order_line.sales_order_id = sales_order.id
WHERE sales_order.customer_id = 2
ORDER BY sales_order.id, order_line.id;
-----------------------------------------------------------
The query planner decides to use the following nested loop:
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Incremental Sort (cost=26.90..16020.06 rows=144955 width=8)
Sort Key: sales_order.id, order_line.id
Presorted Key: sales_order.id
-> Nested Loop (cost=0.70..4593.99 rows=144955 width=8)
-> Index Scan using sales_order_pkey on sales_order (cost=0.28..19.31 rows=79 width=4)
Filter: (customer_id = 2)
-> Index Only Scan using order_line_ukey on order_line (cost=0.42..39.22 rows=1869 width=8)
Index Cond: (sales_order_id = sales_order.id)
-----------------------------------------------------------
As you can see, the planner does detect that the outer loop returns the rows presorted by [sales_order.id]. However, it's unable to detect that the rows returned by the inner loop are also sorted by [sales_order.id] first, and then by [order_line.id].
Why is it? Is it because the planner is designed to always ignore the order of the inner loop, even although it could take advantage of it (for example, because the analysis time rarely is worth it)? Or is there something that I'm missing?
If I'm not mistaken, in this case both index scans seem to be done serially, in an N x M style, so I think the row order would be preserved, Right?
Thank you!
negora