Question

I have the following questions regarding BLOBs in sqlite:

• Does sqlite keep track of sizes of BLOBs?
• I’m guessing that it does, but then, does the length function use it, or does it read the BLOB’s content?
• If sqlite keeps track of the size of the BLOB and length doesn’t use it, is the size accessible via some other functionality?

I’m asking this because I’m wondering if I should implement triggers that set BLOBs’ sizes in additional columns, of if I can obtain the sizes dynamically without the performance hit of sqlite reading the BLOBs.

Answer 1

From the source:

** In an SQLite index record, the serial type is stored directly before
** the blob of data that it corresponds to. In a table record, all serial
** types are stored at the start of the record, and the blobs of data at
** the end. Hence these functions allow the caller to handle the
** serial-type and data blob seperately.
**
** The following table describes the various storage classes for data:
**
**   serial type        bytes of data      type
**   --------------     ---------------    ---------------
**      0                     0            NULL
**      1                     1            signed integer
**      2                     2            signed integer
**      3                     3            signed integer
**      4                     4            signed integer
**      5                     6            signed integer
**      6                     8            signed integer
**      7                     8            IEEE float
**      8                     0            Integer constant 0
**      9                     0            Integer constant 1
**     10,11                               reserved for expansion
**    N>=12 and even       (N-12)/2        BLOB
**    N>=13 and odd        (N-13)/2        text

In other words, the blob size is in the serial, and it’s length is simply “(serial_type-12)/2”.
This serial is stored before the actual blob, so you don’t need to read the blob to get its size.
Call sqlite3_blob_open and then sqlite3_blob_bytes to get this value.