On 2019-11-13 17:37, blackfalconsoftw...@outlook.com [firebird-support] wrote: > Your statement, which may be true, does not sit well with a previous > statement that states that a VARCHAR field of 1000 characters is > stored in the table with 1000 characters. > > Your statement is suggesting a null length (until the field is > updated) with two bytes for an actual length, which is how VARCHAR > fields work in all other databases to my knowledge. Otherwise, to > follow the previous statement, a Firebird table could have a VARCHAR > field for 1000 characters, be stored as such with the initial > storage-info bytes holding the actual length, which would be 1000. > When the field is updated to lets say 20 characters of data, the field > would still have an actual size of 1000 characters but the > storage-info would be 20. > > How does this make any sense?
A record on-disk is a compressed version of the in-memory image of the data of a row. A VARCHAR(1000) in server memory has 2 bytes for the actual data and 1000 bytes for the data, and 2 bytes padding to make it a multiple of 4. Bytes beyond the actual data length are zeroed-out. When written to disk, a RLE compression is applied, so a VARCHAR(1000) containing 'A' will be something like (in hexadecimal): 03 01 00 41 80 00 80 00 80 00 80 00 80 00 80 00 80 00 97 00 Where 03 means: next 3 bytes literally 01 00 means: actual length 1 41 means: literally A 80 00 means: repeat 128 times 00 (x7) 97 00 means: repeat 105 times 00 These last two give a total of 1001 0x00 bytes (zero-padded value + additional padding to make it a multiple of 4. A CHAR(1000) with value A would be encoded as: 01 41 80 20 80 20 80 20 80 20 80 20 80 20 80 20 99 20 Where: 01 menas: next 1 bytes literally 41 means: literally A 80 20 means: repeat 128 times 20 (space) (x7) 99 20 means: repeat 103 times 20 (space) These last two give a total of 999 0x20 bytes (space-padded value), no additional padding because value is already a multiple of 4. NOTE: I might have some details wrong, but this is how it basically work. The Firebird Internal docoument, section data page also describes this: https://firebirdsql.org/file/documentation/reference_manuals/reference_material/html/fbint-page-5.html Mark
