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Adjust cost model for HashAgg that spills to disk.

Tomas Vondra observed that the IO behavior for HashAgg tends to beworse than for Sort. Penalize HashAgg IO costs accordingly.Also, account for the CPU effort of spilling the tuples and readingthem back.Discussion:https://postgr.es/m/20200906212112.nzoy5ytrzjjodpfh@developmentReviewed-by: Tomas VondraBackpatch-through: 13

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src/backend/optimizer/path
- costsize.c

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`‎src/backend/optimizer/path/costsize.c`

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Original file line number	Diff line number	Diff line change
`@@ -2416,6 +2416,7 @@ cost_agg(Path path, PlannerInfo root,`
`2416`	`2416`	`doublepages;`
`2417`	`2417`	`doublepages_written=0.0;`
`2418`	`2418`	`doublepages_read=0.0;`
	`2419`	`+doublespill_cost;`
`2419`	`2420`	`doublehashentrysize;`
`2420`	`2421`	`doublenbatches;`
`2421`	`2422`	`Sizemem_limit;`
`@@ -2453,9 +2454,21 @@ cost_agg(Path path, PlannerInfo root,`
`2453`	`2454`	`pages=relation_byte_size(input_tuples,input_width) /BLCKSZ;`
`2454`	`2455`	`pages_written=pages_read=pages*depth;`
`2455`	`2456`
	`2457`	`+/*`
	`2458`	`+ * HashAgg has somewhat worse IO behavior than Sort on typical`
	`2459`	`+ * hardware/OS combinations. Account for this with a generic penalty.`
	`2460`	`+ */`
	`2461`	`+pages_read *=2.0;`
	`2462`	`+pages_written *=2.0;`
	`2463`	`+`
`2456`	`2464`	`startup_cost+=pages_written*random_page_cost;`
`2457`	`2465`	`total_cost+=pages_written*random_page_cost;`
`2458`	`2466`	`total_cost+=pages_read*seq_page_cost;`
	`2467`	`+`
	`2468`	`+/* account for CPU cost of spilling a tuple and reading it back */`
	`2469`	`+spill_cost=depthinput_tuples2.0*cpu_tuple_cost;`
	`2470`	`+startup_cost+=spill_cost;`
	`2471`	`+total_cost+=spill_cost;`
`2459`	`2472`	`}`
`2460`	`2473`
`2461`	`2474`	`/*`

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Commita547e68

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`‎src/backend/optimizer/path/costsize.c`

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