Vladimir Ozerov created IGNITE-7948:
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Summary: 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
Assignee: Vladimir Ozerov
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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