When there is a single locker in a tuple, we can just store the locking info

in the tuple itself. We do this by storing the locker's Xid in XMAX, and

setting infomask bits specifying the locking strength. There is one exception

here: since infomask space is limited, we do not provide a separate bit

for SELECT FOR SHARE, so we have to use the extended info in a MultiXact in

that case. (The other cases, SELECT FOR UPDATE and SELECT FOR KEY SHARE, are

presumably more commonly used due to being the standards-mandated locking

mechanism, or heavily used by the RI code, so we want to provide fast paths

for those.)

setting infomask bits specifying the locking strength. See "Infomask Bits"

below for details on the bit patterns we use.

MultiXacts

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