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Improve programmer docs for simplehash and dynahash.

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When reading the code it's not obvious when one should prefer dynahash
over simplehash and vice-versa, so, for programmer-friendliness, add
comments to inform that decision.

Show sample simplehash method signatures.

Author: James Coleman <jtc331@gmail.com>
Discussion: https://postgr.es/m/CAAaqYe_dOF39gAJ8rL-a3YO3Qo96MHMRQ2whFjK5ZcU6YvMQSA%40mail.gmail.com
This commit is contained in:
Thomas Munro
2020-08-01 12:16:15 +12:00
parent c79aed4f79
commit 84c0e4b9bc
2 changed files with 80 additions and 5 deletions
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12
src/backend/utils/hash/dynahash.c
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@@ -1,7 +1,7 @@
/*-------------------------------------------------------------------------
*
* dynahash.c
* dynamic hash tables
* dynamic chained hash tables
*
* dynahash.c supports both local-to-a-backend hash tables and hash tables in
* shared memory. For shared hash tables, it is the caller's responsibility
@@ -41,6 +41,16 @@
* function must be supplied; comparison defaults to memcmp() and key copying
* to memcpy() when a user-defined hashing function is selected.
*
* Compared to simplehash, dynahash has the following benefits:
*
* - It supports partitioning, which is useful for shared memory access using
* locks.
* - Shared memory hashes are allocated in a fixed size area at startup and
* are discoverable by name from other processes.
* - Because entries don't need to be moved in the case of hash conflicts, has
* better performance for large entries
* - Guarantees stable pointers to entries.
*
* Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*