CODEWITHSUNDEEP
sqlprestoamazon-athena
Spandan Brahmbhatt
Spandan Brahmbhatt

Reputation: 4054

Avoid computing same value multiple times in Presto

Sample Table :

╔═══════════════╗
║ dummy_data    ║
╠═══════════════╣
║ XXX_1234_YYYY ║
║ XXX_5678_YYYY ║
║ XXX_9101_YYYY ║
╚═══════════════╝

Desired output :

╔═══════════════╦═════════════╦═════════════╗
║  dummy_data   ║ with_prefix ║ with_suffix ║
╠═══════════════╬═════════════╬═════════════╣
║ XXX_1234_YYYY ║ Pre_1234    ║ 1234_Suf    ║
║ XXX_5678_YYYY ║ Pre_5678    ║ 5678_Suf    ║
║ XXX_9101_YYYY ║ Pre_9101    ║ 9101_Suf    ║
╚═══════════════╩═════════════╩═════════════╝

Currently I am trying to write a Presto Query to use on Amazon Athena,

select
    dummy_data,
    'Pre_' || split(dummy_data,'_')[2] as with_prefix,
    split(dummy_data,'_')[2] || '_Suf' as with_suffix
from dummy_table

the above query causes split(dummy_data,'_')[2] to be computed two times. If there a way to avoid the double computation ? Also does Presto figure out that its the same computation and avoids recomputing ?

I am trying to avoid using sub-query or With clause.

Upvotes: 0

Views: 360

Answers (1)

Piotr Findeisen
Piotr Findeisen

Reputation: 20770

Currently Presto does not de-decuplicate sub-expression calculations. I created feature request for this: https://github.com/prestosql/presto/issues/1070. You can see that the expression is evaluated more than once by checking the EXPLAIN output (see the issue for example).

As you know, you can force de-duplication by using an inline view:

SELECT
    dummy_data,
    'Pre_' || sub_data  AS with_prefix,
    sub_data || '_Suf' AS with_suffix
FROM (
    SELECT
        dummy_data, 
        split(dummy_data,'_')[2] AS sub_data
    FROM (VALUES 'XXX_1234_YYYY') t(dummy_data)
);

Upvotes: 2

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