Alvaro Herrera escribió:
> An INSERT wal record is:
>
> typedef struct xl_heap_insert
> {
> xl_heaptid target; /* inserted tuple id */
> bool all_visible_cleared; /* PD_ALL_VISIBLE was cleared */
> /* xl_heap_header & TUPLE DATA FOLLOWS AT END OF STRUCT */
> } xl_heap_insert;
Oops. xl_heaptid is not 6 bytes, but instead:
typedef struct xl_heaptid
{
RelFileNode node;
ItemPointerData tid;
} xl_heaptid;
typedef struct RelFileNode
{
Oid spcNode;
Oid dbNode;
Oid relNode;
} RelFileNode; /* 12 bytes */
typedef struct ItemPointerData
{
BlockIdData ip_blkid;
OffsetNumber ip_posid;
}; /* 6 bytes */
typedef struct BlockIdData
{
uint16 bi_hi;
uint16 bi_lo;
} BlockIdData; /* 4 bytes */
typedef uint16 OffsetNumber;
There's purposely no alignment padding anywhere, so xl_heaptid totals 22 bytes.
Therefore,
> So the fixed part is just 22 bytes + 5 bytes; tuple data follows that.
> So adding four more bytes could indeed be significant (but by how much,
> depends on the size of the tuple data).
4 extra bytes on top of 27 is 14% of added overhead (considering only
the xlog header.)
--
Álvaro Herrera http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
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