ashughes ashughes - 4 months ago 51
Android Question

Retrieve large blob from Android sqlite database

I stored chunks of binary data (protobufs) in the sqlite database of an Android app without realizing that Android's

Cursor
can only hold a maximum of 1MB of data. I now know that I should have stored these binary blobs in files and only referenced the files in the sqlite database entries.

I need to upgrade the database (the app has been in use for a while) in order to move these binary chunks to files. The problem is that some user's data may have already exceeded the 1MB limit and I'm not able to retrieve it from the database (accessing the resulting
Cursor
for a single row that contains a large blob throws an
IllegalStateException: Couldn't read row 0, col 0 from CursorWindow. Make sure the Cursor is initialize before accessing data from it
).

How do I retrieve the binary blobs that are greater than the 1MB
Cursor
limit that have been stored in the sqlite database?

CL. CL.
Answer

You can read large blobs in pieces. First find out which ones need this treatment:

SELECT id, length(blobcolumn) FROM mytable WHERE length(blobcolumn) > 1000000

and then read chunks with substr:

SELECT substr(blobcolumn,       1, 1000000) FROM mytable WHERE id = 123
SELECT substr(blobcolumn, 1000001, 1000000) FROM mytable WHERE id = 123
...

You could also compile your own copy of SQLite and access either the BLOB stream I/O functions or the normal query functions of the C API with the NDK, but that would be too complex in this case.