Since SQLite can store any data in any column, my question - how SQLite engine knows which is which?
Lets say a table has column A and there are 2 values 123 and '#$%'. How these two are stored (and I would like to know byte-by-byte layout to understand)?
What happens with these values if I do select * from table where a > 0 ?
Obviously the engine needs to figure out each value type in column A.. How exactly does it do?
Does it store a type marker with each value so it examines marker on select? or does it evaluates type every time it retrieves a value?
There is a type marker called serial type. It is a variable length integer encoding both the type and the length of the value.
NULL, 0, 1, and '' (empty text) need 1 byte (1 byte for the serial type, no value)
Integers needs 2 to 9 bytes (1 byte serial type, 1 to 8 byte(s) value), so 123 needs 2 bytes.
For text values, the serial type may expand to multiple bytes. Assuming UTF-8, '#$%' needs 4 bytes (1 byte serial type because the encoded length is <= 57)
There is more details in the Database File Format.
Related
I'm reading on this topic, and I just found that IP headers, or even TCP headers, can be of variable length.
I thought routers will use a known length, divided into segments of a known format, to do whatever they do. So, how can you know which segements of bits mean what, if the length of the header is variable?
The way I understand it, is that the Options field at the end of the header determines the total length of the header (present if the Length field value > 5). The Length field has a minimum value of 5 (no options present) and a maximum of 15. This indicates a header length from 20 bytes (5 × 32 bits) to 60 bytes (15 × 32 bits). As all other fields apart from the Options field have a fixed length, it is clear where the header ends and where a given field is supposed to be. The Option field in itself also includes a Option length field for determining the length of Option data.
trying to finish variable locations parser for hlsl, but can't find a way to get structure member size, in this question How to reflect information about hlsl struct members? it was recommended to use offset, but it won't give the actual size of member because of 16-byte packing, so if structure consists of float2 and float4 - it's total size is 32 bytes, offset of second member is 16 bytes, but real size of the first member is 8 bytes and not 16 bytes, i understand that in terms of passed memory there is 16 bytes for that variable and only 8 used, but i have "safety check" that when some value is put in stream in location of this variable - it is of the same size (in case of checking if value at least of variable size - placing it in stream can override data of next variables, and in case of checking if value is of variable size or less - it can lead to hard-to-track bugs if i forget to pass enough data for variable), so is there a way to get the real size of structure member?
I am trying to determine what the correct value would be to input a number into an SQli query. I have been able to deduce (I believe) that the database is using little endian by playing with the hex values. For example:
To modify a value to 255, I am having to use 00000000FF000000.
For values 1-255 this has been fine, it has worked every time.
My question is, for the values 265 and 275, which in hex is converted to 109 and 113, how would I position the numbers within this 16 digit series?
0000000010900000 does not work and returns a value of 36880
Likewise, shifting the 109 to the left instead, returns a value of 9
I am using http://www.scadacore.com/field-applications/programming-calculators/online-hex-converter/ to try and determine the correct value, using the little endian boxes.
The value string seem to be a structure or an array consisting of 16-bit values in little-endian format.
You can see this by examining the values this way:
for example:
00000000FF000000
can be broken down into 16-bit values:
0000 0000 FF00 0000
And when byte-order for 0xff00 is swapped we get 0xff, ie. 255.
For this value:
0000000010900000
we get:
0000 0000 1090 0000
When byte-order is swapped to 0x9010 we get 36880, as in the result.
You don't state why there is a 16-char limit that needs to be fulfilled, but in general - this seem to be part of a structure or an 16-bit array rather than a 64-bit value (which is normally represented by 8 bytes, or 16 chars) but since the value then should have been at either end it is likely not a 64-bit value.
This is normally not something one can guess automatically and would require you know the structure or array definition (talking about binary arrays). One can of course use a pattern to find values matching a range using various positions, width and byte-order, but I cannot see you would benefit in this case compared to finding out the structure/array format (above I guess 4x 16-bit values, but it could just as well have been 1x32-bit, 1x16-bit and 2x8-bit etc.).
My question is, for the values 265 and 275, which in hex is converted to 109 and 113, how would I position the numbers within this 16 digit series?
To fill the binary array you would place them at position 4 (0-base) as 16-bit values in little-endian order.
You can use the DataView for this as it has built-in support for endianess.
Example
Converting value 275 and placing it in the array with correct endianess and position:
var arr = new Uint8Array(8);
var view = new DataView(arr.buffer);
// set value in correct byte-order and position
view.setUint16(4, 275, true); // pos, value, little-endian
// show result (convert to hex-string)
for (var str = "", i = 0; i < arr.length; i++) str += pad(arr[i].toString(16));
console.log(str);
function pad(s) {return s.length < 2 ? "0"+s : s}; // helper: pads to format 0x00
This produces the string:
0000000013010000
Which if we use the same method as earlier becomes:
0000 0000 1301 0000
And then byte-order swap 0x1301 it becomes 0x0113, or 275 as expected.
Blobs are just an array of bytes; the database does not modify their contents or their order in any way.
The endianness (or any other way in which the bytes are to be interpreted) is determined only by the application that writes and reads the blob.
I am trying to find a straightforward way to store negative values in EEPROM, integer values ranging from -20 to 20. I have been using EEPROM.write and EEPROM.read functions to store strings one character at a time, but I am having trouble with negative numbers. I figure I only need one byte for this value.
It's just matter of number representation. You just have to use correct data types to print or use:
Version 1: int8_t data = EEPROM.read(addr);
Version 2:
byte data = EEPROM.read(addr);
Serial.print((int8_t)data);
EEPROM.write can be used directly with int8_t: EEPROM.write(int8_value);
Or, if you wan't int, put/get methods can be used for it (even for structs containing POD types only or so)
I have a question about the data types in sqlite3.
As a value of SQLITE_INTEGER can be stored in 1, 2, 3, 4, 6, or 8 bytes depending on the magnitude of the value, if I only know that a column in SQlite database stores SQLITE_INTEGER, how can I know a value in this column is 4 bytes or 6-8 bytes integer, or which one should be used to get the value, sqlite3_column_int() or sqlite3_column_int64()?
Can I use sqlite3_column_bytes() in this case? but according to the documentation, sqlite3_column_bytes() is primarily used for TEXT or BLOB.
Thanks!
When SQLite steps into a record, all integer values are expanded to 64 bits.
sqlite3_column_int() returns the lower 32 bits of that value without checking for overflows.
When you call sqlite3_column_bytes(), SQLite will convert the value to text, and return the number of characters.
You cannot know how large an integer value is before reading it.
Afterwards, you can check the list in the record format documentation for the smallest possible format for that value, but if you want to be sure that integer values are never truncated to 32 bits, you have to always use sqlite3_column_int64(), or ensure that large values get never written to the DB in the first place (if that is possible).