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Commit228ed43

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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 parent7679eff commit228ed43

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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
@@ -5696,10 +5696,12 @@ power_var(NumericVar *base, NumericVar *exp, NumericVar *result)
56965696
staticvoid
56975697
power_var_int(NumericVar*base,intexp,NumericVar*result,intrscale)
56985698
{
5699+
unsignedintmask;
56995700
boolneg;
57005701
NumericVarbase_prod;
57015702
intlocal_rscale;
57025703

5704+
/* Handle some common special cases, as well as corner cases */
57035705
switch (exp)
57045706
{
57055707
case0:
@@ -5733,23 +5735,43 @@ power_var_int(NumericVar *base, int exp, NumericVar *result, int rscale)
57335735
* pattern of exp. We do the multiplications with some extra precision.
57345736
*/
57355737
neg= (exp<0);
5736-
exp=Abs(exp);
5738+
mask=Abs(exp);
57375739

57385740
local_rscale=rscale+MUL_GUARD_DIGITS*2;
57395741

57405742
init_var(&base_prod);
57415743
set_var_from_var(base,&base_prod);
57425744

5743-
if (exp&1)
5745+
if (mask&1)
57445746
set_var_from_var(base,result);
57455747
else
57465748
set_var_from_var(&const_one,result);
57475749

5748-
while ((exp >>=1)>0)
5750+
while ((mask >>=1)>0)
57495751
{
57505752
mul_var(&base_prod,&base_prod,&base_prod,local_rscale);
5751-
if (exp&1)
5753+
if (mask&1)
57525754
mul_var(&base_prod,result,result,local_rscale);
5755+
5756+
/*
5757+
* When abs(base) > 1, the number of digits to the left of the decimal
5758+
* point in base_prod doubles at each iteration, so if exp is large we
5759+
* could easily spend large amounts of time and memory space doing the
5760+
* multiplications. But once the weight exceeds what will fit in
5761+
* int16, the final result is guaranteed to overflow (or underflow, if
5762+
* exp < 0), so we can give up before wasting too many cycles.
5763+
*/
5764+
if (base_prod.weight>SHRT_MAX||result->weight>SHRT_MAX)
5765+
{
5766+
/* overflow, unless neg, in which case result should be 0 */
5767+
if (!neg)
5768+
ereport(ERROR,
5769+
(errcode(ERRCODE_NUMERIC_VALUE_OUT_OF_RANGE),
5770+
errmsg("value overflows numeric format")));
5771+
zero_var(result);
5772+
neg= false;
5773+
break;
5774+
}
57535775
}
57545776

57555777
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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