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
