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Why has setting a large jdbc fetch size a bad impact on Performance ?

Posted on 2008-11-19
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Last Modified: 2013-12-19
Hi,

Im doing some tuning with the jdbc driver of oracle and i remarked, that setting a large fetch size is not always a good way to improve the performance of a client.
Can anyone explain me why is this so  !

Thank you in advance.
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Question by:finaris
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sujith80 earned 1000 total points
ID: 22992747
Large size means more data, leads to more network traffic and causes latency. Client need to cache more data.
Optimal fetch sizes are in the ranges of 100-500. (Of course varies with the volume of data being fetched.)
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by:Tolomir
Tolomir earned 1000 total points
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It's all about memory allocation. The fetched data has to be stored in memory before it can be processed.

In the worst case you fetch more data before your client is able to process the input, do this 10 times and you eat up all available memory.

So a better way is always to fetch just so much data that can be processed before the next data arrives. It's like traffic jam, as long as there is just so much traffic the needle eye can handle all cars can drive fast, with more cars they have to wait before they can get through, the result is traffic jam.
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by:finaris
ID: 22993425
I still did not understand the drawback of setting the fetchSize as large as possible.

Suppose we have a table with 1000 rows and i want to display these rows in a graphical table.

Alternative A : Set fetchSize to 1000. This leads to copy the rows to the memory in 1 RDBMS access.
Alternative B  : Set fetchSize to 10.  This leads to copy the rows to the memory in 100 RDBMS access.


Under which circumstance can be alternative B less performant as alternative A ?
     
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by:Tolomir
ID: 22993559
set it to 250 to have 4 fetches - leading to the best performance
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by:finaris
ID: 22993773
@Tolomir: why not one fetch ? 4 fetches are less performant, since for each fetch a request has to be sent to the RDBMS.
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by:Tolomir
ID: 22994259
The fetch means no traffic (maybe 1 kb), but the result can be easier handled. See again my traffic jam example, above...
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by:sujith80
ID: 23001505
>> 4 fetches are less performant, since for each fetch a request has to be sent to the RDBMS.

Four fetch calls are made to the RDBMS, true. But it doesnt establish 4 connections and it doesn't go through 4 query processing steps. It uses the same opened cursor.

You are aksed to carry 200kg of something from point A to point B, do you think dragging the whole thing from A to B get you there faster or 10 trips of 20kg each?
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