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Commit26f8a46

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Fix power_var_int() for large integer exponents.
The code for raising a NUMERIC value to an integer power wasn't verycareful about large powers. It got an outright wrong answer for anexponent of INT_MIN, due to failure to consider overflow of the Abs(exp)operation; which is fixable by using an unsigned rather than signedexponent value after that point. Also, even though the number ofiterations of the power-computation loop is pretty limited, it's easy forthe repeated squarings to result in ridiculously enormous intermediatevalues, which can take unreasonable amounts of time/memory to process,or even overflow the internal "weight" field and so produce a wrong answer.We can forestall misbehaviors of that sort by bailing out as soon as theweight value exceeds what will fit in int16, since then the final answermust overflow (if exp > 0) or underflow (if exp < 0) the packed numericformat.Per off-list report from Pavel Stehule. Back-patch to all supportedbranches.
1 parent030fd8a commit26f8a46

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3 files changed

+54
-4
lines changed

3 files changed

+54
-4
lines changed

‎src/backend/utils/adt/numeric.c

Lines changed: 26 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -5440,10 +5440,12 @@ power_var(NumericVar *base, NumericVar *exp, NumericVar *result)
54405440
staticvoid
54415441
power_var_int(NumericVar*base,intexp,NumericVar*result,intrscale)
54425442
{
5443+
unsignedintmask;
54435444
boolneg;
54445445
NumericVarbase_prod;
54455446
intlocal_rscale;
54465447

5448+
/* Handle some common special cases, as well as corner cases */
54475449
switch (exp)
54485450
{
54495451
case0:
@@ -5477,23 +5479,43 @@ power_var_int(NumericVar *base, int exp, NumericVar *result, int rscale)
54775479
* pattern of exp. We do the multiplications with some extra precision.
54785480
*/
54795481
neg= (exp<0);
5480-
exp=Abs(exp);
5482+
mask=Abs(exp);
54815483

54825484
local_rscale=rscale+MUL_GUARD_DIGITS*2;
54835485

54845486
init_var(&base_prod);
54855487
set_var_from_var(base,&base_prod);
54865488

5487-
if (exp&1)
5489+
if (mask&1)
54885490
set_var_from_var(base,result);
54895491
else
54905492
set_var_from_var(&const_one,result);
54915493

5492-
while ((exp >>=1)>0)
5494+
while ((mask >>=1)>0)
54935495
{
54945496
mul_var(&base_prod,&base_prod,&base_prod,local_rscale);
5495-
if (exp&1)
5497+
if (mask&1)
54965498
mul_var(&base_prod,result,result,local_rscale);
5499+
5500+
/*
5501+
* When abs(base) > 1, the number of digits to the left of the decimal
5502+
* point in base_prod doubles at each iteration, so if exp is large we
5503+
* could easily spend large amounts of time and memory space doing the
5504+
* multiplications. But once the weight exceeds what will fit in
5505+
* int16, the final result is guaranteed to overflow (or underflow, if
5506+
* exp < 0), so we can give up before wasting too many cycles.
5507+
*/
5508+
if (base_prod.weight>SHRT_MAX||result->weight>SHRT_MAX)
5509+
{
5510+
/* overflow, unless neg, in which case result should be 0 */
5511+
if (!neg)
5512+
ereport(ERROR,
5513+
(errcode(ERRCODE_NUMERIC_VALUE_OUT_OF_RANGE),
5514+
errmsg("value overflows numeric format")));
5515+
zero_var(result);
5516+
neg= false;
5517+
break;
5518+
}
54975519
}
54985520

54995521
free_var(&base_prod);

‎src/test/regress/expected/numeric.out

Lines changed: 19 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -1390,3 +1390,22 @@ select div(12345678901234567890, 123) * 123 + 12345678901234567890 % 123;
13901390
12345678901234567890
13911391
(1 row)
13921392

1393+
--
1394+
-- Test code path for raising to integer powers
1395+
--
1396+
select 10.0 ^ -2147483648 as rounds_to_zero;
1397+
rounds_to_zero
1398+
--------------------
1399+
0.0000000000000000
1400+
(1 row)
1401+
1402+
select 10.0 ^ -2147483647 as rounds_to_zero;
1403+
rounds_to_zero
1404+
--------------------
1405+
0.0000000000000000
1406+
(1 row)
1407+
1408+
select 10.0 ^ 2147483647 as overflows;
1409+
ERROR: value overflows numeric format
1410+
select 117743296169.0 ^ 1000000000 as overflows;
1411+
ERROR: value overflows numeric format

‎src/test/regress/sql/numeric.sql

Lines changed: 9 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -828,3 +828,12 @@ select 12345678901234567890 % 123;
828828
select12345678901234567890/123;
829829
select div(12345678901234567890,123);
830830
select div(12345678901234567890,123)*123+12345678901234567890 %123;
831+
832+
--
833+
-- Test code path for raising to integer powers
834+
--
835+
836+
select10.0 ^-2147483648as rounds_to_zero;
837+
select10.0 ^-2147483647as rounds_to_zero;
838+
select10.0 ^2147483647as overflows;
839+
select117743296169.0 ^1000000000as overflows;

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