diff options
Diffstat (limited to 'src/include/lib/simplehash.h')
| -rw-r--r-- | src/include/lib/simplehash.h | 50 |
1 files changed, 48 insertions, 2 deletions
diff --git a/src/include/lib/simplehash.h b/src/include/lib/simplehash.h index 9622131ede6..031a377da84 100644 --- a/src/include/lib/simplehash.h +++ b/src/include/lib/simplehash.h @@ -125,6 +125,7 @@ #define SH_ITERATE SH_MAKE_NAME(iterate) #define SH_ALLOCATE SH_MAKE_NAME(allocate) #define SH_FREE SH_MAKE_NAME(free) +#define SH_ESTIMATE_SPACE SH_MAKE_NAME(estimate_space) #define SH_STAT SH_MAKE_NAME(stat) /* internal helper functions (no externally visible prototypes) */ @@ -242,7 +243,10 @@ SH_SCOPE void SH_START_ITERATE_AT(SH_TYPE * tb, SH_ITERATOR * iter, uint32 at); /* <element> *<prefix>_iterate(<prefix>_hash *tb, <prefix>_iterator *iter) */ SH_SCOPE SH_ELEMENT_TYPE *SH_ITERATE(SH_TYPE * tb, SH_ITERATOR * iter); -/* void <prefix>_stat(<prefix>_hash *tb */ +/* size_t <prefix>_estimate_space(double nentries) */ +SH_SCOPE size_t SH_ESTIMATE_SPACE(double nentries); + +/* void <prefix>_stat(<prefix>_hash *tb) */ SH_SCOPE void SH_STAT(SH_TYPE * tb); #endif /* SH_DECLARE */ @@ -305,7 +309,7 @@ SH_SCOPE void SH_STAT(SH_TYPE * tb); /* * Compute allocation size for hashtable. Result can be passed to - * SH_UPDATE_PARAMETERS. + * SH_UPDATE_PARAMETERS. (Keep SH_ESTIMATE_SPACE in sync with this!) */ static inline uint64 SH_COMPUTE_SIZE(uint64 newsize) @@ -1069,6 +1073,47 @@ SH_ITERATE(SH_TYPE * tb, SH_ITERATOR * iter) } /* + * Estimate the amount of space needed for a hashtable with nentries entries. + * Return SIZE_MAX if that's too many entries. + * + * nentries is "double" because this is meant for use by the planner, + * which typically works with double rowcount estimates. So we'd need to + * clamp to integer somewhere and that might as well be here. We do expect + * the value not to be NaN or negative, else the result will be garbage. + */ +SH_SCOPE size_t +SH_ESTIMATE_SPACE(double nentries) +{ + uint64 size; + uint64 space; + + /* scale request by SH_FILLFACTOR, as SH_CREATE does */ + nentries = nentries / SH_FILLFACTOR; + + /* fail if we'd overrun SH_MAX_SIZE entries */ + if (nentries >= SH_MAX_SIZE) + return SIZE_MAX; + + /* should be safe to convert to uint64 */ + size = (uint64) nentries; + + /* supporting zero sized hashes would complicate matters */ + size = Max(size, 2); + + /* round up size to the next power of 2, that's how bucketing works */ + size = pg_nextpower2_64(size); + + /* calculate space needed for ->data */ + space = ((uint64) sizeof(SH_ELEMENT_TYPE)) * size; + + /* verify that allocation of ->data is possible on this platform */ + if (space >= SIZE_MAX / 2) + return SIZE_MAX; + + return (size_t) space + sizeof(SH_TYPE); +} + +/* * Report some statistics about the state of the hashtable. For * debugging/profiling purposes only. */ @@ -1195,6 +1240,7 @@ SH_STAT(SH_TYPE * tb) #undef SH_ITERATE #undef SH_ALLOCATE #undef SH_FREE +#undef SH_ESTIMATE_SPACE #undef SH_STAT /* internal function names */ |