November 13, 2016
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ISSFQ:
Athena supports only two kinds of Select statement
Athena supports only two kinds of select statements?
Can anyone explain the difference between them?
SELECT *
FROM table
WHERE column = value;
WHERE (column = value OR column2 = value);
A:
A select statement is executed in three steps:
Initialize the query result.
Partition the query.
Execute the query plan on the partitioned query.
STEP 1:
The query is partitioned into rows/records, each consisting of one or more columns.
The expressions in WHERE are evaluated on each partition. If the partition matches the WHERE clause, then the partition is included in the output. For example, here are two rows from the same table:
In the first row, there is no condition on column. The WHERE clause is true, so the whole row is selected.
In the second row, there is a condition on column, and column is false. The WHERE clause is false, so the whole row is discarded.
STEP 2:
The WHERE clause is translated into a logical AND between the partition columns, and each partition is evaluated separately. If the condition is true for the whole row, then the partition is returned to the final output.
For example, consider the same query above. Here are the results after step 2:
As you can see, there are two partitions in the output. The first partition matched the WHERE clause, so was returned in the output. The second partition did not match the WHERE clause, so was not returned in the output.
STEP 3:
The last step is to run the physical execution plan on the partitioned query. The plan is a tree structure, where each leaf is a record, each branch is a join, and each inner node is an operator that calculates a function (e.g. +, -, /, etc.).
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