@ -30,11 +30,12 @@
* dynahash . c provides support for these types of lookup keys :
*
* 1. Null - terminated C strings ( truncated if necessary to fit in keysize ) ,
* compared as though by strcmp ( ) . This is the default behavior .
* compared as though by strcmp ( ) . This is selected by specifying the
* HASH_STRINGS flag to hash_create .
*
* 2. Arbitrary binary data of size keysize , compared as though by memcmp ( ) .
* ( Caller must ensure there are no undefined padding bits in the keys ! )
* This is selected by specifying HASH_BLOBS flag to hash_create .
* This is selected by specifying the HASH_BLOBS flag to hash_create .
*
* 3. More complex key behavior can be selected by specifying user - supplied
* hashing , comparison , and / or key - copying functions . At least a hashing
@ -47,8 +48,8 @@
* locks .
* - Shared memory hashes are allocated in a fixed size area at startup and
* are discoverable by name from other processes .
* - Because entries don ' t need to be moved in the case of hash conflicts , has
* better performance for large entries
* - Because entries don ' t need to be moved in the case of hash conflicts ,
* dynahash has better performance for large entries .
* - Guarantees stable pointers to entries .
*
* Portions Copyright ( c ) 1996 - 2020 , PostgreSQL Global Development Group
@ -316,6 +317,28 @@ string_compare(const char *key1, const char *key2, Size keysize)
* * info : additional table parameters , as indicated by flags
* flags : bitmask indicating which parameters to take from * info
*
* The flags value * must * include HASH_ELEM . ( Formerly , this was nominally
* optional , but the default keysize and entrysize values were useless . )
* The flags value must also include exactly one of HASH_STRINGS , HASH_BLOBS ,
* or HASH_FUNCTION , to define the key hashing semantics ( C strings ,
* binary blobs , or custom , respectively ) . Callers specifying a custom
* hash function will likely also want to use HASH_COMPARE , and perhaps
* also HASH_KEYCOPY , to control key comparison and copying .
* Another often - used flag is HASH_CONTEXT , to allocate the hash table
* under info - > hcxt rather than under TopMemoryContext ; the default
* behavior is only suitable for session - lifespan hash tables .
* Other flags bits are special - purpose and seldom used , except for those
* associated with shared - memory hash tables , for which see ShmemInitHash ( ) .
*
* Fields in * info are read only when the associated flags bit is set .
* It is not necessary to initialize other fields of * info .
* Neither tabname nor * info need persist after the hash_create ( ) call .
*
* Note : It is deprecated for callers of hash_create ( ) to explicitly specify
* string_hash , tag_hash , uint32_hash , or oid_hash . Just set HASH_STRINGS or
* HASH_BLOBS . Use HASH_FUNCTION only when you want something other than
* one of these .
*
* Note : for a shared - memory hashtable , nelem needs to be a pretty good
* estimate , since we can ' t expand the table on the fly . But an unshared
* hashtable can be expanded on - the - fly , so it ' s better for nelem to be
@ -323,11 +346,19 @@ string_compare(const char *key1, const char *key2, Size keysize)
* large nelem will penalize hash_seq_search speed without buying much .
*/
HTAB *
hash_create ( const char * tabname , long nelem , HASHCTL * info , int flags )
hash_create ( const char * tabname , long nelem , const HASHCTL * info , int flags )
{
HTAB * hashp ;
HASHHDR * hctl ;
/*
* Hash tables now allocate space for key and data , but you have to say
* how much space to allocate .
*/
Assert ( flags & HASH_ELEM ) ;
Assert ( info - > keysize > 0 ) ;
Assert ( info - > entrysize > = info - > keysize ) ;
/*
* For shared hash tables , we have a local hash header ( HTAB struct ) that
* we allocate in TopMemoryContext ; all else is in shared memory .
@ -370,28 +401,43 @@ hash_create(const char *tabname, long nelem, HASHCTL *info, int flags)
* Select the appropriate hash function ( see comments at head of file ) .
*/
if ( flags & HASH_FUNCTION )
{
Assert ( ! ( flags & ( HASH_BLOBS | HASH_STRINGS ) ) ) ;
hashp - > hash = info - > hash ;
}
else if ( flags & HASH_BLOBS )
{
Assert ( ! ( flags & HASH_STRINGS ) ) ;
/* We can optimize hashing for common key sizes */
Assert ( flags & HASH_ELEM ) ;
if ( info - > keysize = = sizeof ( uint32 ) )
hashp - > hash = uint32_hash ;
else
hashp - > hash = tag_hash ;
}
else
hashp - > hash = string_hash ; /* default hash function */
{
/*
* string_hash used to be considered the default hash method , and in a
* non - assert build it effectively still is . But we now consider it
* an assertion error to not say HASH_STRINGS explicitly . To help
* catch mistaken usage of HASH_STRINGS , we also insist on a
* reasonably long string length : if the keysize is only 4 or 8 bytes ,
* it ' s almost certainly an integer or pointer not a string .
*/
Assert ( flags & HASH_STRINGS ) ;
Assert ( info - > keysize > 8 ) ;
hashp - > hash = string_hash ;
}
/*
* If you don ' t specify a match function , it defaults to string_compare if
* you used string_hash ( either explicitly or by default ) and to memcmp
* otherwise .
* you used string_hash , and to memcmp otherwise .
*
* Note : explicitly specifying string_hash is deprecated , because this
* might not work for callers in loadable modules on some platforms due to
* referencing a trampoline instead of the string_hash function proper .
* Just let it default , eh ?
* Specify HASH_STRINGS instead .
*/
if ( flags & HASH_COMPARE )
hashp - > match = info - > match ;
@ -505,16 +551,9 @@ hash_create(const char *tabname, long nelem, HASHCTL *info, int flags)
hctl - > dsize = info - > dsize ;
}
/*
* hash table now allocates space for key and data but you have to say how
* much space to allocate
*/
if ( flags & HASH_ELEM )
{
Assert ( info - > entrysize > = info - > keysize ) ;
hctl - > keysize = info - > keysize ;
hctl - > entrysize = info - > entrysize ;
}
/* remember the entry sizes, too */
hctl - > keysize = info - > keysize ;
hctl - > entrysize = info - > entrysize ;
/* make local copies of heavily-used constant fields */
hashp - > keysize = hctl - > keysize ;
@ -593,10 +632,6 @@ hdefault(HTAB *hashp)
hctl - > dsize = DEF_DIRSIZE ;
hctl - > nsegs = 0 ;
/* rather pointless defaults for key & entry size */
hctl - > keysize = sizeof ( char * ) ;
hctl - > entrysize = 2 * sizeof ( char * ) ;
hctl - > num_partitions = 0 ; /* not partitioned */
/* table has no fixed maximum size */
Tom Lane
5 years ago