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https://issues.apache.org/jira/browse/IMPALA-12373?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17787132#comment-17787132
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Csaba Ringhofer commented on IMPALA-12373:
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>I think we don't need NULL termination so we can store actually 11 chars with
>libc++'s technique.
In case the StringValue is inside a tuple, it may be possible to store even
more chars in the tuple.
The idea is to reserve some bytes before the StringValue, and if based on the
last bit it is a small string, but length is > 11, then we could assume that
the string starts len - 11 bytes before the address of StringValue. This could
speed things up a bit in case we have stats about avg/max string length, as we
the number of extra bytes could be chosen to minimize waste.
> Implement Small String Optimization for StringValue
> ---------------------------------------------------
>
> Key: IMPALA-12373
> URL: https://issues.apache.org/jira/browse/IMPALA-12373
> Project: IMPALA
> Issue Type: Improvement
> Reporter: Zoltán Borók-Nagy
> Assignee: Zoltán Borók-Nagy
> Priority: Major
> Labels: Performance
> Attachments: small_string.cpp
>
>
> Implement Small String Optimization for StringValue.
> Current memory layout of StringValue is:
> {noformat}
> char* ptr; // 8 byte
> int len; // 4 byte
> {noformat}
> For small strings with size up to 8 we could store the string contents in the
> bytes of the 'ptr'. Something like that:
> {noformat}
> union {
> char* ptr;
> char small_buf[sizeof(ptr)];
> };
> int len;
> {noformat}
> Many C++ string implementations use the {{Small String Optimization}} to
> speed up work with small strings. For example:
> {code:java}
> Microsoft STL, libstdc++, libc++, Boost, Folly.{code}
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