[jira] [Updated] (IGNITE-7948) SQL: read only necessary fields into the row when possible

["Maxim Muzafarov (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/IGNITE-7948?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Maxim Muzafarov updated IGNITE-7948:
------------------------------------
    Fix Version/s:     (was: 2.8)
                   2.9

> SQL: read only necessary fields into the row when possible
> ----------------------------------------------------------
>
>                 Key: IGNITE-7948
>                 URL: https://issues.apache.org/jira/browse/IGNITE-7948
>             Project: Ignite
>          Issue Type: Task
>          Components: sql
>            Reporter: Vladimir Ozerov
>            Priority: Major
>             Fix For: 2.9
>
>          Time Spent: 10m
>  Remaining Estimate: 0h
>
> When H2 row is read, we always fill it with data eagerly through link materialization. Materialization is performed under page "read lock" what guarantees row-level consistency. This may lead to excessive memory pressure due to memory copying. For example, consider a class with 50 fields and a query which reads only 2 of them. 48 other fields will be copied without a reason. Lazy initialization is not an option because it will only defer memcpy, but not eliminate it. 
> Instead we can try using H2. It passes {{TableFilter}} class to some of index access methods*. We can analyze this class and create the list of required fields. Then we can read these fields under read lock from offheap and put them to the row.
> In addition to saved memcpy this could give us more benefits:
> 1) No more need for field cache ({{GridH2KeyValueRowOnheap#valCache}})
> 2) No more need to read {{_VER}} column and possibly {{_KEY}} or {{_VAL}}
> But there are a number of drawbacks as well. E.g. it is impossible to read strings from offheap efficiently, so queries with VARCHAR will definitely suffer from this change.
> \* {{org.h2.index.Index#find(org.h2.table.TableFilter, org.h2.result.SearchRow, org.h2.result.SearchRow)}}



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