HashTable KeySize

On Fri, Oct 04, 2019 at 05:06:47PM +0530, Natarajan R wrote:

typedef struct HashTableKey
{
Oid dbId; // 4 bytes
int64 productid; // 8 bytes
}HashTableKey; (total size - 12 bytes)

typedef struct HashTableEntry
{
HashTableKey key;
ProductInfo *pdt;
}HashTableEntry;

HASHCTL hashInfo;
hashInfo.keysize = sizeof(HashTableKey);
hashInfo.entrysize = sizeof(HashTableEntry);
SampleHashTable = ShmemInitHash("productid vs product struct HashTable",
size, size, &hashInfo, HASH_ELEM | HASH_SHARED_MEM | HASH_BLOBS);

while printing keysize: elog(LOG,"Keysize = %d",sizeof(HashTableKey));

I am getting Keysize = 16, How? what should i need to do inorder to have
keysize = 12

That's likely due to alignment. The second field is a 64-bit value will
be aligned at 8-byte boundary, so in memory the struct will look like
this:

dbId -- 4 bytes
padding -- 4 bytes
productId -- 8 bytes

See

https://en.wikipedia.org/wiki/Data_structure_alignment

and there's also a tool to show the memory layout:

https://linux.die.net/man/1/pahole

regards

--
Tomas Vondra http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services