Server Responses to Client Communication - Result Set Packets
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A large number of queries produce a result set. Some examples are SELECT, SHOW, CHECK,REPAIR, andEXPLAIN. Any time the expected information from a query is more than a simple status report, a result set is returned.
The result set consists of a sequence of packets. First, the server sends information about the fields with a call toProtocol::send_fields()in sql/protocol.cc in version 4.1 and later. In the older versions, the function is calledsend_fields()and is found in sql/net_pkg.cc. This stage produces the following sequence of packets:
Server Responses
A packet with the body consisting of the standard field-length specifier sequence. However, this time, the meaning of the number is different. It indicates the number of fields in the result set.
A group of field description packets (see the upcoming explanation for the format description), one for each field, in the field order of the result set.
Optional element. If present, contains the default value of the field in the standard field data format.
Table 4-13. Option flags in server’s result set packets
Bit macro
Hexadecimal bit value
Description
NOT_NULL_FLAG
0x0001
The field value cannot be NULL(it is declared with the NOT NULL attribute).
PRI_KEY_FLAG
0x0002
The field is a part of the primary key.
UNIQUE_KEY_FLAG
0x0004
The field is a part of a unique key.
MULTIPLE_KEY_ FLAG
0x0008
The field is a part of some non-unique key.
BLOB_FLAG
0x0010
The field is a BLOBor TEXT.
UNSIGNED_FLAG
0x0020
The field was declared with the UNSIGNEDattribute, which has the same meaning as the unsignedkeyword in C.
ZEROFILL_FLAG
0x0040
The field has been declared with the ZEROFILLattribute, which tells the server to pad the numeric types with leading zeros in the output to fit the specified field length.
BINARY_FLAG
0x0080
The field has been declared with the BINARYattribute, which tells the server to compare strings byte-for-byte in a case-sensitive manner.
ENUM_FLAG
0x0100
The field is an ENUM.
AUTO_INCREMENT_ FLAG
0x0200
The field has been declared with the AUTO_INCREMENTattribute, which enables the automatic generation of primary key values when a new record is inserted.
TIMESTAMP_FLAG
0x0400
The field is a timestamp.
SET_FLAG
0x0800
The field is a SET.
NUM_FLAG
0x8000
Used with cursors in version 4.1 to indicate that the field is numeric.
Following the field definition sequence of packets, the server sends the actual rows of data, one packet per row. Each row data packet consists of a sequence of values stored in the standard field data format. When reporting the result of a regular query (sent withCOM_QUERY), the field data is converted to the string format. When using a prepared statement (COM_PREPARE), the field data is sent in its native format with the low byte first.
After all of the data rows have been sent, the packet sequence is terminated with anEOFpacket.
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