Fix realtime get of nested fields with synthetic source (#119575) #120247
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Today, for get-from-translog operations, we only need to reindex the root document into an in-memory Lucene, as the _source is stored in the root document and is sufficient. However, synthesizing the source for nested fields requires both the root document and its child documents. This causes realtime-get operations (as well as update and update-by-query operations) to miss nested fields. Another issue is that the translog operation is reindexed lazily during get-from-translog operations. As a result, two realtime-get operations can return slightly different outputs: one reading from the translog and the other from Lucene. This change resolves both issues. However, addressing the second issue can degrade the performance of realtime-get and update operations. If slight inconsistencies are acceptable, the translog operation should be reindexed lazily instead. Closes elastic#119553
dnhatn
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:StorageEngine/Mapping
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Today, for get-from-translog operations, we only need to reindex the root document into an in-memory Lucene, as the _source is stored in the root document and is sufficient. However, synthesizing the source for nested fields requires both the root document and its child documents. This causes realtime-get operations (as well as update and update-by-query operations) to miss nested fields.
Another issue is that the translog operation is reindexed lazily during get-from-translog operations. As a result, two realtime-get operations can return slightly different outputs: one reading from the translog and the other from Lucene.
This change resolves both issues. However, addressing the second issue can degrade the performance of realtime-get and update operations. If slight inconsistencies are acceptable, the translog operation should be reindexed lazily instead.
Closes #119553