As part of the database migration to support threaded receipts, there is
a possible window in between
`73/08thread_receipts_non_null.sql.postgres` removing the original
unique constraints on `receipts_linearized` and `receipts_graph` and the
`reeipts_linearized_unique_index` and `receipts_graph_unique_index`
background updates from `72/08thread_receipts.sql` completing where
the unique constraints on `receipts_linearized` and `receipts_graph` are
missing. Any emulated upserts on these tables must therefore be
performed with a lock held, otherwise duplicate rows can end up in the
tables when there are concurrent emulated upserts.

Note that emulated upserts no longer happen by default on sqlite, since
the minimum supported version of sqlite supports native upserts by
default now.

Signed-off-by: Sean Quah <[email protected]>

Before creating the `receipts_graph_unique_index` and
`receipts_linearized_unique_index` unique indexes, we have to clean up
any duplicate receipts that may have crept in due to
#14406.

Signed-off-by: Sean Quah <[email protected]>

Signed-off-by: Sean Quah <[email protected]>

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…_constraint_background_update

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