Details
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Task
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Status: Open (View Workflow)
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Major
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Resolution: Unresolved
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None
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None
Description
Suppose we're running
t1 full outer join t2 on on_expr |
and we use join buffer.
The first step is to read rows from t1 and put them into join buffer:
join_buffer
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t1.recordX, match_flag=false
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t1.recordY, match_flag=false
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t1.recordZ, match_flag=false
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Suppose the join buffer is large enough and all records from t1 fit there.
Then we do joining. I am providing existing LEFT JOIN algorithm, comments show how we can add FULL JOIN easily:
for each record R2 in t2 { |
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for each record R1 from join buffer { |
if (on_expr(R1, R2)) |
emit(R1.t1_record, R2);
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}
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// If the above loop didn't produce anything, we know |
// that R2 didn't have a match. |
// So, we can do: |
// emit(t1.null_complemented_record, R2); |
}
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for each record R1 in join_buffer { |
if (R1.match_flag == false) |
emit(R1.t1_record, t2.null_complemented_record);
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}
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Note that this doesn't require a temporary table, or any extra space in the join buffer.
The only condition is that all the matching rows from the first table fit into the join buffer.
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