Reading execution plan - first row time vs last row time

Dear community,

We see significant difference in explain analyze Actual time in the first line of execution plan and Execution time in the last line of execution plan. What can be the reason?

For example, first line of execution plan:
Gather (cost=1038.49..257627100.42 rows=7943222226 width=104) (actual time=531.925..3673159.806 rows=7943221053 loops=1)

This is 3673159/1000/60 = 61 minutes

And last 2 lines of execution plan:
Planning Time: 3000.399 ms
Execution Time: 21208130.602 ms

This is : 21208130/1000/60= 353 minutes

What happened in 353 - 61 = 292 minutes? We speculate that disk writes. Query possibly written cca 0,75TB of data, by create table as select query.

S pozdravem
Ján Pecsők

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