diff options
Diffstat (limited to 'src/include/postgres.h')
| -rw-r--r-- | src/include/postgres.h | 37 |
1 files changed, 19 insertions, 18 deletions
diff --git a/src/include/postgres.h b/src/include/postgres.h index dfce01bf4b1..082c75b0935 100644 --- a/src/include/postgres.h +++ b/src/include/postgres.h @@ -54,11 +54,11 @@ */ /* - * struct varatt_external is a "TOAST pointer", that is, the information needed - * to fetch a Datum stored in an out-of-line on-disk Datum. The data is - * compressed if and only if va_extsize < va_rawsize - VARHDRSZ. This struct - * must not contain any padding, because we sometimes compare pointers using - * memcmp. + * struct varatt_external is a traditional "TOAST pointer", that is, the + * information needed to fetch a Datum stored out-of-line in a TOAST table. + * The data is compressed if and only if va_extsize < va_rawsize - VARHDRSZ. + * This struct must not contain any padding, because we sometimes compare + * these pointers using memcmp. * * Note that this information is stored unaligned within actual tuples, so * you need to memcpy from the tuple into a local struct variable before @@ -74,22 +74,23 @@ typedef struct varatt_external } varatt_external; /* - * Out-of-line Datum thats stored in memory in contrast to varatt_external - * pointers which points to data in an external toast relation. + * struct varatt_indirect is a "TOAST pointer" representing an out-of-line + * Datum that's stored in memory, not in an external toast relation. + * The creator of such a Datum is entirely responsible that the referenced + * storage survives for as long as referencing pointer Datums can exist. * - * Note that just as varatt_external's this is stored unaligned within the - * tuple. + * Note that just as for struct varatt_external, this struct is stored + * unaligned within any containing tuple. */ typedef struct varatt_indirect { struct varlena *pointer; /* Pointer to in-memory varlena */ } varatt_indirect; - /* - * Type of external toast datum stored. The peculiar value for VARTAG_ONDISK - * comes from the requirement for on-disk compatibility with the older - * definitions of varattrib_1b_e where v_tag was named va_len_1be... + * Type tag for the various sorts of "TOAST pointer" datums. The peculiar + * value for VARTAG_ONDISK comes from a requirement for on-disk compatibility + * with a previous notion that the tag field was the pointer datum's length. */ typedef enum vartag_external { @@ -98,9 +99,9 @@ typedef enum vartag_external } vartag_external; #define VARTAG_SIZE(tag) \ - ((tag) == VARTAG_INDIRECT ? sizeof(varatt_indirect) : \ + ((tag) == VARTAG_INDIRECT ? sizeof(varatt_indirect) : \ (tag) == VARTAG_ONDISK ? sizeof(varatt_external) : \ - TrapMacro(true, "unknown vartag")) + TrapMacro(true, "unrecognized TOAST vartag")) /* * These structs describe the header of a varlena object that may have been @@ -132,7 +133,7 @@ typedef struct char va_data[1]; /* Data begins here */ } varattrib_1b; -/* inline portion of a short varlena pointing to an external resource */ +/* TOAST pointers are a subset of varattrib_1b with an identifying tag byte */ typedef struct { uint8 va_header; /* Always 0x80 or 0x01 */ @@ -162,8 +163,8 @@ typedef struct * this lets us disambiguate alignment padding bytes from the start of an * unaligned datum. (We now *require* pad bytes to be filled with zero!) * - * In TOAST datums the tag field in varattrib_1b_e is used to discern whether - * its an indirection pointer or more commonly an on-disk tuple. + * In TOAST pointers the va_tag field (see varattrib_1b_e) is used to discern + * the specific type and length of the pointer datum. */ /* |