Commits on Nov 11, 2021

1. Provide loadBeforeEx …

   loadBeforeEx is like loadBefore, but simpler, provides better
   information for object delete records and can be more efficiently
   implemented by many storages: zopefoundation/ZODB#323
   
   On RelStorage loadBefore is currently implemented via 3 SQL queries:
   
     1) check whether object record exists at all
     2) retrieve object state
     3) retrieve serial of next object revision
   
   Compared to that loadBeforeEx is implemented via only one SQL query "2"
   from the above - "retrieve object state". It is exactly the same query
   that loadBefore uses and after the patch loadBefore actually invokes
   loadBeforeEx for step 2.
   
   This change was outlined in
   
   zopefoundation/ZODB#318 (comment) and
   zopefoundation/ZODB#318 (comment)
   
   and as explained in the first link this patch is also semantically coupled with
   
   zodb#484
   
   This patch passes tests with both ZODB5 and with ZODB5+zopefoundation/ZODB#323:
   
   - when ran with ZODB5 it verifies that loadBefore implementation does
     not become broken.
   
   - when ran with ZODB5+zopefoundation/ZODB#323 it
     verifies that loadBeforeEx implementation is correct.
   
   For tests to pass with
   ZODB5+zopefoundation/ZODB#323 we also need
   zopefoundation/zc.zlibstorage#11 because without
   that fix zc.zlibstorage does not decompress data on loadBeforeEx.

   

   navytux committed Nov 11, 2021
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