Postgresql Limit 1 causing large number of loops - postgresql

Question

We have a transactions table, approx. 10m rows and growing. Each customer we have specifies many rules which group certain transactions together based on locations, associated products, sale customer, etc. Off of these rules we produce reports each night which allows them to see the price the customer is paying for products vs their purchase prices from different price lists, these lists are changing daily and each date on the transaction we have to either find their set yearly price or the price that was effective at the date of the transaction.
These price lists can change historically and do all the time as do new historic transactions which are added so within each financial year we have to continue to regenerate these reports.
We are having a problem with the two types of price list/price joins we have to do. The first is on the set yearly price list.
I have removed the queries which bring the transactions in and put into a table called transaction_data_6787.
EXPLAIN analyze
SELECT *
FROM transaction_data_6787 t
inner JOIN LATERAL
(
SELECT p."Price"
FROM "Prices" p
INNER JOIN "PriceLists" pl on p."PriceListId" = pl."Id"
WHERE (pl."CustomerId" = 20)
AND (pl."Year" = 2020)
AND (pl."PriceListTypeId" = 2)
AND p."ProductId" = t.product_id
limit 1
) AS prices ON true
Nested Loop (cost=0.70..133877.20 rows=5394 width=165) (actual time=0.521..193.638 rows=5394 loops=1) -> Seq Scan on transaction_data_6787 t (cost=0.00..159.94 rows=5394 width=145) (actual time=0.005..0.593 rows=5394 loops=1) -> Limit (cost=0.70..24.77 rows=1 width=20) (actual time=0.035..0.035 rows=1 loops=5394)
-> Nested Loop (cost=0.70..24.77 rows=1 width=20) (actual time=0.035..0.035 rows=1 loops=5394)
-> Index Scan using ix_prices_covering on "Prices" p (cost=0.42..8.44 rows=1 width=16) (actual time=0.006..0.015 rows=23 loops=5394)
Index Cond: (("ProductId" = t.product_id))
-> Index Scan using ix_pricelists_covering on "PriceLists" pl (cost=0.28..8.30 rows=1 width=12) (actual time=0.001..0.001 rows=0 loops=122443)
Index Cond: (("Id" = p."PriceListId") AND ("CustomerId" = 20) AND ("PriceListTypeId" = 2))
Filter: ("Year" = 2020)
Rows Removed by Filter: 0 Planning Time: 0.307 ms Execution Time: 193.982 ms
If I remove the LIMIT 1, the execution time drops to 3ms and the 122443 loops on ix_pricelists_covering don't happen. The reason we are doing a lateral join is the price query is dynamically built and sometimes when not joining on the annual price list we join on the effective price lists. This looks like the below:
EXPLAIN analyze
SELECT *
FROM transaction_data_6787 t
inner JOIN LATERAL
(
SELECT p."Price"
FROM "Prices" p
INNER JOIN "PriceLists" pl on p."PriceListId" = pl."Id"
WHERE (pl."CustomerId" = 20)
AND (pl."PriceListTypeId" = 1)
AND p."ProductId" = t.product_id
and pl."ValidFromDate" <= t.transaction_date
ORDER BY pl."ValidFromDate" desc
limit 1
) AS prices ON true
This is killing our performance, some queries are taking 20 seconds plus when we don't order by date desc/limit 1 it completes in ms but we could get duplicate prices back.
We are happy to rewrite if a better way of joining the most recent record. We have thousands of price lists and 100k or prices and there could be 100s if not 1000s of effective prices for each transaction and we need to ensure we get the one which was most recently effective for a product at the date of the transaction.
I have found if I denormalise the price lists/prices into a single table and add an index with ValidFromDate DESC it seems to eliminate the loops but I am hesitant to denormalise and have to maintain that data, these reports can be run adhoc as well as batch jobs and we would have to maintain that data in real time.
Updated Explain/Analyze:
I've added below the query which joins on prices which need to get the most recently effective for the transaction date. I see now that when <= date clause and limit 1 is removed it's actually spinning up multiple workers which is why it seems faster.
I am still seeing the slower query doing a large number of loops, 200k+ (when limit 1/<= date is included).
Maybe the better question is what can we do instead of a lateral join which will allow us to join the effective prices for transactions in the most efficient/performant way possible. I am hoping to avoid denormalising and maintainint that data but if it is the only way we'll do it. If there is a way to rewrite this and not denormalise then I'd really appreciate any insight.
Nested Loop (cost=14.21..76965.60 rows=5394 width=10) (actual time=408.948..408.950 rows=0 loops=1)
Output: t.transaction_id, pr."Price"
Buffers: shared hit=688022
-> Seq Scan on public.transaction_data_6787 t (cost=0.00..159.94 rows=5394 width=29) (actual time=0.018..0.682 rows=5394 loops=1)
Output: t.transaction_id
Buffers: shared hit=106
-> Limit (cost=14.21..14.22 rows=1 width=10) (actual time=0.075..0.075 rows=0 loops=5394)
Output: pr."Price", pl."ValidFromDate"
Buffers: shared hit=687916
-> Sort (cost=14.21..14.22 rows=1 width=10) (actual time=0.075..0.075 rows=0 loops=5394)
Output: pr."Price", pl."ValidFromDate"
Sort Key: pl."ValidFromDate" DESC
Sort Method: quicksort Memory: 25kB
Buffers: shared hit=687916
-> Nested Loop (cost=0.70..14.20 rows=1 width=10) (actual time=0.074..0.074 rows=0 loops=5394)
Output: pr."Price", pl."ValidFromDate"
Inner Unique: true
Buffers: shared hit=687916
-> Index Only Scan using ix_prices_covering on public."Prices" pr (cost=0.42..4.44 rows=1 width=10) (actual time=0.007..0.019 rows=51 loops=5394)
Output: pr."ProductId", pr."ValidFromDate", pr."Id", pr."Price", pr."PriceListId"
Index Cond: (pr."ProductId" = t.product_id)
Heap Fetches: 0
Buffers: shared hit=17291
-> Index Scan using ix_pricelists_covering on public."PriceLists" pl (cost=0.28..8.30 rows=1 width=8) (actual time=0.001..0.001 rows=0 loops=273678)
Output: pl."Id", pl."Name", pl."CustomerId", pl."ValidFromDate", pl."PriceListTypeId"
Index Cond: ((pl."Id" = pr."PriceListId") AND (pl."CustomerId" = 20) AND (pl."PriceListTypeId" = 1))
Filter: (pl."ValidFromDate" <= t.transaction_date)
Rows Removed by Filter: 0
Buffers: shared hit=670625
Planning Time: 1.254 ms
Execution Time: 409.088 ms
Gather (cost=6395.67..7011.99 rows=68 width=10) (actual time=92.481..92.554 rows=0 loops=1)
Output: t.transaction_id, pr."Price"
Workers Planned: 2
Workers Launched: 2
Buffers: shared hit=1466 read=2
-> Hash Join (cost=5395.67..6005.19 rows=28 width=10) (actual time=75.126..75.129 rows=0 loops=3)
Output: t.transaction_id, pr."Price"
Inner Unique: true
Hash Cond: (pr."PriceListId" = pl."Id")
Join Filter: (pl."ValidFromDate" <= t.transaction_date)
Rows Removed by Join Filter: 41090
Buffers: shared hit=1466 read=2
Worker 0: actual time=64.707..64.709 rows=0 loops=1
Buffers: shared hit=462
Worker 1: actual time=72.545..72.547 rows=0 loops=1
Buffers: shared hit=550 read=1
-> Merge Join (cost=5374.09..5973.85 rows=3712 width=18) (actual time=26.804..61.492 rows=91226 loops=3)
Output: t.transaction_id, t.transaction_date, pr."Price", pr."PriceListId"
Merge Cond: (pr."ProductId" = t.product_id)
Buffers: shared hit=1325 read=2
Worker 0: actual time=17.677..51.590 rows=83365 loops=1
Buffers: shared hit=400
Worker 1: actual time=24.995..59.395 rows=103814 loops=1
Buffers: shared hit=488 read=1
-> Parallel Index Only Scan using ix_prices_covering on public."Prices" pr (cost=0.42..7678.38 rows=79544 width=29) (actual time=0.036..12.136 rows=42281 loops=3)
Output: pr."ProductId", pr."ValidFromDate", pr."Id", pr."Price", pr."PriceListId"
Heap Fetches: 0
Buffers: shared hit=989 read=2
Worker 0: actual time=0.037..9.660 rows=36873 loops=1
Buffers: shared hit=285
Worker 1: actual time=0.058..13.459 rows=47708 loops=1
Buffers: shared hit=373 read=1
-> Sort (cost=494.29..507.78 rows=5394 width=29) (actual time=9.037..14.700 rows=94555 loops=3)
Output: t.transaction_id, t.product_id, t.transaction_date
Sort Key: t.product_id
Sort Method: quicksort Memory: 614kB
Worker 0: Sort Method: quicksort Memory: 614kB
Worker 1: Sort Method: quicksort Memory: 614kB
Buffers: shared hit=336
Worker 0: actual time=6.608..12.034 rows=86577 loops=1
Buffers: shared hit=115
Worker 1: actual time=8.973..14.598 rows=107126 loops=1
Buffers: shared hit=115
-> Seq Scan on public.transaction_data_6787 t (cost=0.00..159.94 rows=5394 width=29) (actual time=0.020..2.948 rows=5394 loops=3)
Output: t.transaction_id, t.product_id, t.transaction_date
Buffers: shared hit=318
Worker 0: actual time=0.017..2.078 rows=5394 loops=1
Buffers: shared hit=106
Worker 1: actual time=0.027..2.976 rows=5394 loops=1
Buffers: shared hit=106
-> Hash (cost=21.21..21.21 rows=30 width=8) (actual time=0.145..0.145 rows=35 loops=3)
Output: pl."Id", pl."ValidFromDate"
Buckets: 1024 Batches: 1 Memory Usage: 10kB
Buffers: shared hit=53
Worker 0: actual time=0.137..0.138 rows=35 loops=1
Buffers: shared hit=18
Worker 1: actual time=0.149..0.150 rows=35 loops=1
Buffers: shared hit=18
-> Bitmap Heap Scan on public."PriceLists" pl (cost=4.59..21.21 rows=30 width=8) (actual time=0.067..0.114 rows=35 loops=3)
Output: pl."Id", pl."ValidFromDate"
Recheck Cond: (pl."CustomerId" = 20)
Filter: (pl."PriceListTypeId" = 1)
Rows Removed by Filter: 6
Heap Blocks: exact=15
Buffers: shared hit=53
Worker 0: actual time=0.068..0.108 rows=35 loops=1
Buffers: shared hit=18
Worker 1: actual time=0.066..0.117 rows=35 loops=1
Buffers: shared hit=18
-> Bitmap Index Scan on "IX_PriceLists_CustomerId" (cost=0.00..4.58 rows=41 width=0) (actual time=0.049..0.049 rows=41 loops=3)
Index Cond: (pl."CustomerId" = 20)
Buffers: shared hit=8
Worker 0: actual time=0.053..0.054 rows=41 loops=1
Buffers: shared hit=3
Worker 1: actual time=0.048..0.048 rows=41 loops=1
Buffers: shared hit=3
Planning Time: 2.236 ms
Execution Time: 92.814 ms