[postgis-tickets] [PostGIS] #4261: Revise index lock mode in spatial_index_read_extent

[PostGIS]

#4261: Revise index lock mode in spatial_index_read_extent
-------------------------+---------------------------
 Reporter:  Algunenano   |      Owner:  pramsey
     Type:  enhancement  |     Status:  new
 Priority:  medium       |  Milestone:  PostGIS 3.0.0
Component:  postgis      |    Version:  trunk
 Keywords:               |
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 From IRC, use `andehhh` has reported a locking problem related with an
 external process having an `AccessShareLock` to the spatial index and a
 call to `st_estimatedextent`.

 As `st_estimatedextent` is using `AccessExclusiveLock` it will block until
 the other function finishes even though it's only using it for reading.

 We could revise whether that level of exclusive access to the index is
 really needed or we can make do with less "exclusivity".

 From [1]:
 {{{
 Index access methods must handle concurrent updates of the index by
 multiple processes. The core PostgreSQL system obtains AccessShareLock on
 the index during an index scan, and RowExclusiveLock when updating the
 index (including plain VACUUM). Since these lock types do not conflict,
 the access method is responsible for handling any fine-grained locking it
 might need. An exclusive lock on the index as a whole will be taken only
 during index creation, destruction, or REINDEX.
 }}}

 I understand that the operation we are doing in
 `spatial_index_read_extent` is equivalent to an index scan but I don't
 think the perf gain it's worth the added complexity of dealing with
 concurrent modifications. We could avoid that using "just" a
 `ExclusiveLock`, which allows concurrent read locks but blocks any
 modifications.

 Am I missing something?

 Some refs:

 [1] https://www.postgresql.org/docs/11/index-locking.html

 [2] https://www.postgresql.org/docs/11/index-scanning.html

 [3] https://www.postgresql.org/docs/11/explicit-locking.html

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Ticket URL: <https://trac.osgeo.org/postgis/ticket/4261>
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