How to limit number of items I fetch from DynamoDB with PartiQL? - php

Question

I have this query which I run in PHP:
$result = $client->executeStatement([
'Limit' => 1,
'Statement' => "SELECT * FROM transactions WHERE completed = 0",
]);
I have tried using query function as well but that too supports Limit which is not actually a limit.
$result = $db->query(array(
'TableName' => 'transactions',
'IndexName' => 'completed-index',
'Count' => 1,
'Limit' => 1,
'ScannedCount' => 1,
'KeyConditions' => array(
'completed' => array(
'AttributeValueList' => array(
array('N' => '1')
),
'ComparisonOperator' => 'EQ'
),
),
));
According to their documentation, Limit doesnt necessarily mean a number of matching items:
The maximum number of items to evaluate (not necessarily the number of
matching items). If DynamoDB processes the number of items up to the
limit while processing the results, it stops the operation and returns
the matching values up to that point
Can anyone tell me if there is an actual way to limit the number of rows returned just like we do in SQL databases?

Answer 1

You can only limit how much data is read from disk (pre-filter), not how much is returned (post-filter).
DynamoDB never allows you to request for unbounded work. If DynamoDB allowed you to ask for just 1 row with a filter condition that never matched anything, that would potentially need to read the full database trying to find that 1 row. Behavior like that is what causes relational databases to have issues at scale.
Now, if you're not specifying a filter condition and your query is fully indexed, the amount read will match the amount returned, so the limit should act pretty much like you'd want.
Otherwise you might have to make repeated calls to page the results until you get as many rows as you want.