On 2/26/23 15:30, jack...@gmail.com wrote: > here are the source codes from src/include/access/htup_details.h. > /* > * information stored in t_infomask: > */ > #define HEAP_HASNULL0x0001/* has null attribute(s) */ > #define HEAP_HASVARWIDTH0x0002/* has variable-width attribute(s) */ > #define HEAP_HASEXTERNAL0x0004/* has external stored attribute(s) */ > #define HEAP_HASOID_OLD0x0008/* has an object-id field */ > #define HEAP_XMAX_KEYSHR_LOCK0x0010/* xmax is a key-shared locker */ > #define HEAP_COMBOCID0x0020/* t_cid is a combo CID */ > #define HEAP_XMAX_EXCL_LOCK0x0040/* xmax is exclusive locker */ > #define HEAP_XMAX_LOCK_ONLY0x0080/* xmax, if valid, is only a locker */ > > And I can't understand these attrs:
I suggest you try something like 'git grep HEAP_HASEXTERNAL' which shows you where the flag is used, which should tell you what it means. These short descriptions generally assume you know enough about the internals. > 1. external stored attribute(s), what is this? can you give a create > statement to show me? external = value stored in a TOAST table > 2. xmax is a key-shared locker/exclusive locker/only a locker, so how > you use this? can you give me a scenario? > let me try to explain it: > if there is a txn is trying to read this heaptuple, > the HEAP_XMAX_KEYSHR_LOCK bit will be set to 1. > if there is a txn is trying to delete/update this heaptuple, > the HEAP_XMAX_EXCL_LOCK bit will be set to 1. > but for HEAP_XMAX_LOCK_ONLY, I can't understand. > And another thought is that these three bit can have only one to be set > 1 at most. I believe HEAP_XMAX_LOCK_ONLY means the xmax transaction only locked the tuple, without deleting/updating it. > 3. t_cid is a combo CID? what's a CID? give me an example please. CID means "command ID" i.e. sequential ID assigned to commands in a single session (for visibility checks, so that a query doesn't see data deleted by earlier commands in the same session). See src/backend/utils/time/combocid.c for basic explanation of what "combo CID" is. regards -- Tomas Vondra EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company
