Once a connection to a database server has been successfully established, the functions described here are used to perform SQL queries and commands.
PQexec Submit a command to
the server and wait for the result.
PGresult *PQexec(PGconn *conn,
const char *query);
Returns a PGresult pointer
or possibly a NULL pointer. A non-NULL pointer will
generally be returned except in out-of-memory conditions or
serious errors such as inability to send the command to the
backend. If a NULL is returned, it should be treated like a
PGRES_FATAL_ERROR result. Use
PQerrorMessage to get more
information about the error.
The PGresult structure
encapsulates the result returned by the backend. libpq application programmers should be careful
to maintain the PGresult
abstraction. Use the accessor functions below to get at the
contents of PGresult. Avoid
directly referencing the fields of the PGresult structure because they are subject
to change in the future. (Beginning in PostgreSQL 6.4, the definition of struct
PGresult is not even provided in
libpq-fe.h. If you have old code that
accesses PGresult fields directly,
you can keep using it by including libpq-int.h too, but you are encouraged to fix
the code soon.)
PQresultStatus Returns the
result status of the command.
ExecStatusType PQresultStatus(const PGresult *res)
PQresultStatus can return
one of the following values:
PGRES_EMPTY_QUERY -- The string sent to the backend was empty.
PGRES_COMMAND_OK -- Successful completion of a command returning no data
PGRES_TUPLES_OK -- The query successfully executed
PGRES_COPY_OUT -- Copy Out (from server) data transfer started
PGRES_COPY_IN -- Copy In (to server) data transfer started
PGRES_BAD_RESPONSE -- The server's response was not understood
PGRES_NONFATAL_ERROR
PGRES_FATAL_ERROR
If the result status is PGRES_TUPLES_OK, then the routines described below can be used to retrieve the rows returned by the query. Note that a SELECT command that happens to retrieve zero rows still shows PGRES_TUPLES_OK. PGRES_COMMAND_OK is for commands that can never return rows (INSERT, UPDATE, etc.). A response of PGRES_EMPTY_QUERY often exposes a bug in the client software.
PQresStatus Converts the
enumerated type returned by PQresultStatus into a string
constant describing the status code.
char *PQresStatus(ExecStatusType status);
PQresultErrorMessage
returns the error message associated with the query, or an
empty string if there was no error.
char *PQresultErrorMessage(const PGresult *res);
Immediately following a PQexec or PQgetResult call, PQerrorMessage (on the connection) will
return the same string as PQresultErrorMessage (on the result).
However, a PGresult will retain
its error message until destroyed, whereas the connection's
error message will change when subsequent operations are
done. Use PQresultErrorMessage when you want to
know the status associated with a particular PGresult; use PQerrorMessage when you want to know the
status from the latest operation on the connection.
PQclear Frees the storage
associated with the PGresult.
Every query result should be freed via PQclear when it is no longer needed.
void PQclear(PQresult *res);
You can keep a PGresult
object around for as long as you need it; it does not go
away when you issue a new query, nor even if you close the
connection. To get rid of it, you must call PQclear. Failure to do this will result
in memory leaks in the frontend application.
PQmakeEmptyPGresult
Constructs an empty PGresult
object with the given status.
PGresult* PQmakeEmptyPGresult(PGconn *conn, ExecStatusType status);
This is libpq's
internal routine to allocate and initialize an empty
PGresult object. It is exported
because some applications find it useful to generate result
objects (particularly objects with error status)
themselves. If conn is not
NULL and status indicates an error, the connection's
current errorMessage is copied into the PGresult. Note that PQclear should eventually be called on
the object, just as with a PGresult returned by libpq itself.
PQescapeString Escapes a
string for use within an SQL query.
size_t PQescapeString (char *to, const char *from, size_t length);
If you want to include strings that have been received from a source that is not trustworthy (for example, because a random user entered them), you cannot directly include them in SQL queries for security reasons. Instead, you have to quote special characters that are otherwise interpreted by the SQL parser.
PQescapeString performs this
operation. The from points to
the first character of the string that is to be escaped, and
the length parameter counts the
number of characters in this string (a terminating zero byte is
neither necessary nor counted). to shall point to a buffer that is able to
hold at least one more character than twice the value of
length, otherwise the behavior
is undefined. A call to PQescapeString writes an escaped version of
the from string to the
to buffer, replacing special
characters so that they cannot cause any harm, and adding a
terminating zero byte. The single quotes that must surround
PostgreSQL string literals are
not part of the result string.
PQescapeString returns the
number of characters written to to, not including the terminating zero byte.
Behavior is undefined when the to and from
strings overlap.
PQescapeBytea Escapes a binary
string (bytea type) for use within an SQL
query.
unsigned char *PQescapeBytea(unsigned char *from,
size_t from_length,
size_t *to_length);
Certain ASCII characters
must be escaped (but
all characters may be
escaped) when used as part of a bytea
string literal in an SQL
statement. In general, to escape a character, it is converted
into the three digit octal number equal to the decimal
ASCII value, and preceded by
two backslashes. The single quote (') and backslash (\)
characters have special alternate escape sequences. See the
User's Guide for more information.
PQescapeBytea performs this
operation, escaping only the minimally required characters.
The from parameter points to
the first character of the string that is to be escaped, and
the from_length parameter
reflects the number of characters in this binary string (a
terminating zero byte is neither necessary nor counted). The
to_length parameter shall point
to a buffer suitable to hold the resultant escaped string
length. The result string length does not include the
terminating zero byte of the result.
PQescapeBytea returns an
escaped version of the from
parameter binary string, to a caller-provided buffer. The
return string has all special characters replaced so that they
can be properly processed by the PostgreSQL string literal
parser, and the bytea input function. A
terminating zero byte is also added. The single quotes that
must surround PostgreSQL string literals are not part of the
result string.
PQntuples Returns the
number of tuples (rows) in the query result.
int PQntuples(const PGresult *res);
PQnfields Returns the
number of fields (columns) in each row of the query
result.
int PQnfields(const PGresult *res);
PQfname Returns the field
(column) name associated with the given field index. Field
indices start at 0.
char *PQfname(const PGresult *res,
int field_index);
PQfnumber Returns the
field (column) index associated with the given field
name.
int PQfnumber(const PGresult *res,
const char *field_name);
-1 is returned if the given name does not match any field.
PQftype Returns the field
type associated with the given field index. The integer
returned is an internal coding of the type. Field indices
start at 0.
Oid PQftype(const PGresult *res,
int field_index);
You can query the system table pg_type to obtain the name and properties of the various data types. The OIDs of the built-in data types are defined in src/include/catalog/pg_type.h in the source tree.
PQfmod Returns the
type-specific modification data of the field associated
with the given field index. Field indices start at 0.
int PQfmod(const PGresult *res,
int field_index);
PQfsize Returns the size
in bytes of the field associated with the given field
index. Field indices start at 0.
int PQfsize(const PGresult *res,
int field_index);
PQfsize returns the space
allocated for this field in a database tuple, in other
words the size of the server's binary representation of the
data type. -1 is returned if the field is variable
size.
PQbinaryTuples Returns 1
if the PGresult contains binary tuple data, 0 if it
contains ASCII data.
int PQbinaryTuples(const PGresult *res);
Currently, binary tuple data can only be returned by a query that extracts data from a binary cursor.
PQgetvalue Returns a
single field (column) value of one tuple (row) of a
PGresult. Tuple and field
indices start at 0.
char* PQgetvalue(const PGresult *res,
int tup_num,
int field_num);
For most queries, the value returned by PQgetvalue is a null-terminated character
string representation of the attribute value. But if
PQbinaryTuples() is 1, the
value returned by PQgetvalue
is the binary representation of the type in the internal
format of the backend server (but not including the size
word, if the field is variable-length). It is then the
programmer's responsibility to cast and convert the data to
the correct C type. The pointer returned by PQgetvalue points to storage that is part
of the PGresult structure. One
should not modify it, and one must explicitly copy the
value into other storage if it is to be used past the
lifetime of the PGresult
structure itself.
PQgetisnull Tests a field
for a NULL entry. Tuple and field indices start at 0.
int PQgetisnull(const PGresult *res,
int tup_num,
int field_num);
This function returns 1 if the field contains a NULL, 0
if it contains a non-null value. (Note that PQgetvalue will return an empty string,
not a null pointer, for a NULL field.)
PQgetlength Returns the
length of a field (attribute) value in bytes. Tuple and
field indices start at 0.
int PQgetlength(const PGresult *res,
int tup_num,
int field_num);
This is the actual data length for the particular data
value, that is the size of the object pointed to by
PQgetvalue. Note that for
character-represented values, this size has little to do
with the binary size reported by PQfsize.
PQprint Prints out all the
tuples and, optionally, the attribute names to the
specified output stream.
void PQprint(FILE* fout, /* output stream */
const PGresult *res,
const PQprintOpt *po);
struct {
pqbool header; /* print output field headings and row count */
pqbool align; /* fill align the fields */
pqbool standard; /* old brain dead format */
pqbool html3; /* output html tables */
pqbool expanded; /* expand tables */
pqbool pager; /* use pager for output if needed */
char *fieldSep; /* field separator */
char *tableOpt; /* insert to HTML table ... */
char *caption; /* HTML caption */
char **fieldName; /* null terminated array of replacement field names */
} PQprintOpt;
This function was formerly used by psql to print query results, but this is no longer the case and this function is no longer actively supported.
PQcmdStatus Returns the
command status string from the SQL command that generated
the PGresult.
char * PQcmdStatus(const PGresult *res);
PQcmdTuples Returns the
number of rows affected by the SQL command.
char * PQcmdTuples(const PGresult *res);
If the SQL command that generated the PGresult was INSERT, UPDATE or DELETE, this returns a string containing the number of rows affected. If the command was anything else, it returns the empty string.
PQoidValue Returns the
object ID of the inserted row, if the SQL command was an INSERT that inserted
exactly one row into a table that has OIDs. Otherwise,
returns InvalidOid.
Oid PQoidValue(const PGresult *res);
The type Oid and the constant InvalidOid will be defined if you include the libpq header file. They will both be some integer type.
PQoidStatus Returns a
string with the object ID of the inserted row, if the
SQL command was an
INSERT. (The string will be 0 if
the INSERT did not insert exactly one row, or if the target
table does not have OIDs.) If the command was not an
INSERT, returns an empty string.
char * PQoidStatus(const PGresult *res);
This function is deprecated in favor of PQoidValue and is not thread-safe.