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https://issues.apache.org/jira/browse/IMPALA-12373?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17755610#comment-17755610
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Daniel Becker commented on IMPALA-12373:
----------------------------------------
Great idea!
Theoretically, reading the inactive member of a union is UB, so we can't use
{{rep.small_rep.len}} in {{is_small()}}. We could use memcpy to get the last
byte.
Although cppreference says
{code:java}
Many compilers implement, as a non-standard language extension, the ability to
read inactive members of a union.
{code}
https://en.cppreference.com/w/cpp/language/union
> 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
> 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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