Pinning Records
[Files of Records]

Collaboration diagram for Pinning Records:

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Detailed Description

You may pin (force to remain in the buffer pool at a fixed location) portions (no larger than a page) of a record for short periods of time while you operate on them. You may step through a large record pinning a sequence of such portions.

Use of the pin_i requires care. Take care to observe the following constraints:

• You may not operate on the in-buffer-pool copy directly, as the only storage manager knows the format of these data. You may operate on these pinned data through the class pin_i.

• Do not hold page latches (keep a page pinned) for long periods (while a thread sleeps, awaits I/O, or otherwise blocks long-term. Operating system scheduling of threads is not under your control for this purpose). Latches are meant to be short-term. Holding a latch for a long time interferes with other aspects of the storage manager, including buffer-pool cleaning.

• A latch is held by a thread, not by a transaction. Under no circumstances can a pin_i be passed from thread to thread.

• It is dangerous to operate on a record through the static storage manager methods (such as append_rec) while holding records pinned through pin_i. This can lead to invalid pin_i with undefined results when they are used; doing so with concurrent threads in a single transaction can lead to undetectable deadlocks (latch-latch deadlocks, for example).

Classes
class	pin_i
	Pin records in the buffer pool and operate on them. More...

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