Get rid of the multiplier and addend variables in favor of hard-wired
constants. Do the multiply-and-add using uint64 arithmetic, rather
than manually combining several narrower multiplications and additions.
Make _dorand48 return the full-width new random value, and have its
callers use that directly (after suitable masking) rather than
reconstructing what they need from the unsigned short[] representation.
On my machine, this is good for a nearly factor-of-2 speedup of
pg_erand48(), probably mostly from needing just one call of ldexp()
rather than three. The wins for the other functions are smaller
but measurable. While none of the existing call sites are really
performance-critical, a cycle saved is a cycle earned; and besides
the machine code is smaller this way (at least on x86_64).
Patch by me, but the original idea to optimize this by switching
to int64 arithmetic is from Fabien Coelho.
Discussion: https://postgr.es/m/1551.1546018192@sss.pgh.pa.us
POSIX specifies that jrand48() returns a signed 32-bit value (in the
range [-2^31, 2^31)), but our code was returning an unsigned 32-bit
value (in the range [0, 2^32)). This doesn't actually matter to any
existing call site, because they all cast the "long" result to int32
or uint32; but it will doubtless bite somebody in the future.
To fix, cast the arithmetic result to int32 explicitly before the
compiler widens it to long (if widening is needed).
While at it, upgrade this file's far-short-of-project-style comments.
Had there been some peer pressure to document pg_jrand48() properly,
maybe this thinko wouldn't have gotten committed to begin with.
Backpatch to v10 where pg_jrand48() was added, just in case somebody
back-patches a fix that uses it and depends on the standard behavior.
Discussion: https://postgr.es/m/17235.1545951602@sss.pgh.pa.us
This adds a new routine, pg_strong_random() for generating random bytes,
for use in both frontend and backend. At the moment, it's only used in
the backend, but the upcoming SCRAM authentication patches need strong
random numbers in libpq as well.
pg_strong_random() is based on, and replaces, the existing implementation
in pgcrypto. It can acquire strong random numbers from a number of sources,
depending on what's available:
- OpenSSL RAND_bytes(), if built with OpenSSL
- On Windows, the native cryptographic functions are used
- /dev/urandom
Unlike the current pgcrypto function, the source is chosen by configure.
That makes it easier to test different implementations, and ensures that
we don't accidentally fall back to a less secure implementation, if the
primary source fails. All of those methods are quite reliable, it would be
pretty surprising for them to fail, so we'd rather find out by failing
hard.
If no strong random source is available, we fall back to using erand48(),
seeded from current timestamp, like PostmasterRandom() was. That isn't
cryptographically secure, but allows us to still work on platforms that
don't have any of the above stronger sources. Because it's not very secure,
the built-in implementation is only used if explicitly requested with
--disable-strong-random.
This replaces the more complicated Fortuna algorithm we used to have in
pgcrypto, which is unfortunate, but all modern platforms have /dev/urandom,
so it doesn't seem worth the maintenance effort to keep that. pgcrypto
functions that require strong random numbers will be disabled with
--disable-strong-random.
Original patch by Magnus Hagander, tons of further work by Michael Paquier
and me.
Discussion: https://www.postgresql.org/message-id/CAB7nPqRy3krN8quR9XujMVVHYtXJ0_60nqgVc6oUk8ygyVkZsA@mail.gmail.com
Discussion: https://www.postgresql.org/message-id/CAB7nPqRWkNYRRPJA7-cF+LfroYV10pvjdz6GNvxk-Eee9FypKA@mail.gmail.com
The original implementation of TABLESAMPLE modeled the tablesample method
API on index access methods, which wasn't a good choice because, without
specialized DDL commands, there's no way to build an extension that can
implement a TSM. (Raw inserts into system catalogs are not an acceptable
thing to do, because we can't undo them during DROP EXTENSION, nor will
pg_upgrade behave sanely.) Instead adopt an API more like procedural
language handlers or foreign data wrappers, wherein the only SQL-level
support object needed is a single handler function identified by having
a special return type. This lets us get rid of the supporting catalog
altogether, so that no custom DDL support is needed for the feature.
Adjust the API so that it can support non-constant tablesample arguments
(the original coding assumed we could evaluate the argument expressions at
ExecInitSampleScan time, which is undesirable even if it weren't outright
unsafe), and discourage sampling methods from looking at invisible tuples.
Make sure that the BERNOULLI and SYSTEM methods are genuinely repeatable
within and across queries, as required by the SQL standard, and deal more
honestly with methods that can't support that requirement.
Make a full code-review pass over the tablesample additions, and fix
assorted bugs, omissions, infelicities, and cosmetic issues (such as
failure to put the added code stanzas in a consistent ordering).
Improve EXPLAIN's output of tablesample plans, too.
Back-patch to 9.5 so that we don't have to support the original API
in production.
Add a TABLESAMPLE clause to SELECT statements that allows
user to specify random BERNOULLI sampling or block level
SYSTEM sampling. Implementation allows for extensible
sampling functions to be written, using a standard API.
Basic version follows SQLStandard exactly. Usable
concrete use cases for the sampling API follow in later
commits.
Petr Jelinek
Reviewed by Michael Paquier and Simon Riggs
">" should be ">>". This typo results in failure to use all of the bits
of the provided seed.
This might rise to the level of a security bug if we were relying on
srand48 for any security-critical purposes, but we are not --- in fact,
it's not used at all unless the platform lacks srandom(), which is
improbable. Even on such a platform the exposure seems minimal.
Reported privately by Andres Freund.
glibc renders random() thread-safe by wrapping a futex lock around it;
testing reveals that this limits the performance of pgbench on machines
with many CPU cores. Rather than switching to random_r(), which is
only available on GNU systems and crashes unless you use undocumented
alchemy to initialize the random state properly, switch to our built-in
implementation of erand48(), which is both thread-safe and concurrent.
Since the list of reasons not to use the operating system's erand48()
is getting rather long, rename ours to pg_erand48() (and similarly
for our implementations of lrand48() and srand48()) and just always
use those. We were already doing this on Cygwin anyway, and the
glibc implementation is not quite thread-safe, so pgbench wouldn't
be able to use that either.
Per discussion with Tom Lane.
and extend configure to test for it properly instead of hard-wiring
an assumption that everybody but Windows has the rand48 functions.
(We do cheat to the extent of assuming that probing for erand48 will do
for the entire rand48 family.)
erand48() is unused as of this commit, but a followon patch will cause
GEQO to depend on it.
Andres Freund, additional hacking by Tom
