[IMPALA-5560] Consider always storing CHAR() slots inline

• Type: Improvement
• Status: Resolved
• Priority: Minor
• Resolution: Fixed
• Affects Version/s: Impala 2.10.0
• Fix Version/s: Impala 2.10.0
• Component/s: Backend, Frontend
• Target Version: Product Backlog
• Epic Color: ghx-label-1
• Assignee: Tim Armstrong
• Reporter: Tim Armstrong
• Created: 22/Jun/17 17:29
• Updated: 30/Jun/17 23:13
• Resolved: 30/Jun/17 23:13

Description

Currently we have two distinct representations for CHAR slots: one that stores the CHAR(N) inline in the tuple when N <= 128 and another that stores it out-of-line from the tuple when 128 < N <= 255.

We could simplify the code and improve our test coverage by only using a single representation. I think that test coverage for the out-of-line storage has some large gaps so I don't have a high degree of confidence that it is correct in all cases.

The main benefit to the out-of-line storage seems to be that it consumes less space for NULL values. It could also improve cache locality in some cases. These benefits seem marginal though, particularly since CHAR(129+) is probably used only infrequently.

Issue Links

• relates to: IMPALA-3207 Support Char type in codegen
• relates to: IMPALA-5559 Frontend and backend have different notions of which slots are var-len.