A scan with no limit refuses to run when the database holds more records
than this, rather than materializing the lot. One million keys is roughly
70 MB as an R character vector, and more again with their values: enough
headroom for ordinary work, small enough to catch a database that was never
meant to be read in one go.
Value
A length-one numeric vector, 1e6. It is a constant rather than a
function: its value is the number of records above which mdbx_keys() and
mdbx_items() refuse a scan that was given no limit.
Details
The guard applies only when limit is NULL. Any explicit limit is
honoured, including limit = Inf to say "all of them, really".