What is a coalesce stage in Bigquery?

Question

Want to understand what is the coalesce stage in Bigquery and in what cases this stage is seen in the plan. I didn't found information about this stage in bigquery documentation.

Note: I am not using coalesce function in the query.

Following is the screenshot for reference

Answer 1

The coalesce step in the query plan is because you are querying multiple tables. This step is combining the data from each additional table you query. So, each new table, a new coalesce (and join) will be added to combine this data.

Its different than the conditional function COALESCE()

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The point is, each step can directly write only one set of data to a next step. If a new set have to write, this data will be read again, that will be the coalesce step. The following chart represents what happen when 1, 2 or 3 tables a queried, and shows the join and coalesce steps. This is just an exemplification, another operations as cross are been omitted.