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Dodge portability issue (apparent compiler bug) in new tablesample code.

2015年7月25日 23:42:32 +0000 (19:42 -0400)

Some of the older OS X critters in the buildfarm are failing regression,
with symptoms showing that a request for 100% sampling in BERNOULLI or
SYSTEM methods actually gets only around 50% of the table. gdb revealed
that the computation of the "cutoff" number was producing 0x7FFFFFFF
rather than the expected 0x100000000. Inspecting the assembly code,
it looks like gcc is trying to use lrint() instead of rint() and then
fumbling the conversion from long double to uint64. This seems like a
clear compiler bug, but assigning the intermediate result into a plain
double variable works around it, so let's just do that. (Another idea
would be to give up one bit of hash width so that we don't need to use
a uint64 cutoff, but let's see if this is enough.)

src/backend/access/tablesample/bernoulli.c patch | blob | blame | history

src/backend/access/tablesample/system.c patch | blob | blame | history

diff --git a/src/backend/access/tablesample/bernoulli.c b/src/backend/access/tablesample/bernoulli.c

index cf88f95e757b1754da8b4d074c9abfc367560208..ccef4f7f8438828bf6b3de8b6097ca5695335d91 100644 (file)

--- a/src/backend/access/tablesample/bernoulli.c

+++ b/src/backend/access/tablesample/bernoulli.c

@@ -144,6 +144,7 @@ bernoulli_beginsamplescan(SampleScanState *node,

{

BernoulliSamplerData *sampler = (BernoulliSamplerData *) node->tsm_state;

double percent = DatumGetFloat4(params[0]);

+ double dcutoff;

if (percent < 0 || percent > 100 || isnan(percent))