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Update "Iterating tables in batches" docs

Bogdan Denkovych requested to merge bdenkovych-update-iterating_tables_in_batches-docs into master

What does this MR do and why?

In !163461 (comment 2091976406) we noticed that batching query with LEFT JOIN is more than 2x more expensive than without. Since LEFT JOIN is not needed for the each_batch to identify id ranges, we can move LEFT JOIN to yielded relation to make the iteration less expensive.

Query plans

This MR updates an example in our docs of each_batch iteration with LEFT JOIN to suggest less expensive option.

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Edited by Bogdan Denkovych

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