[dataimport] Add SQL Store for faster import - works ONLY with Postgres for now, as it requires "copy from" command - closes #2410822
This store will use:
- copy from for massive insertions.
- execute from for update.
The API of this store is similar to the other stores.
What's new in CubicWeb 3.16?
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New functionnalities
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* Add a new dataimport store (`SQLGenObjectStore`). This store enables a fast
import of data (entity creation, link creation) in CubicWeb, by directly
flushing information in SQL. This may only be used with PostgreSQL, as it
requires the 'COPY FROM' command.
API changes
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Unintrusive API changes
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User interface changes
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