Roman Kondakov created IGNITE-7998:
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             Summary: SQL: Improve MVCC vacuum performance by iterating over 
data pages instead of cache tree. 
                 Key: IGNITE-7998
                 URL: https://issues.apache.org/jira/browse/IGNITE-7998
             Project: Ignite
          Issue Type: Improvement
          Components: sql
            Reporter: Roman Kondakov


At the moment vacuum process uses cache trees to find outdated (dead) entries 
and cache and index trees to cleanup them. It is not efficient due to several 
reasons. For example, we should lock a datapage for each cache tree entry to 
find out if entry is dead.

We can consider a direct iteration over datapages as a possible improvement of  
the vacuum process. Data page iteration prototype demonstrated 5-10 times time 
improvement over the tree iteration.

At first stage we need to implement direct datapages iteration only for 
collecting dead entries links.

At the second stage we need to consider removing links to dead entries from 
index pages directly. In other words, we need to efficiently remove batches of 
dead links from indexes without traversing cache and index tree one dead link 
by one.



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