PostgreSQL 7.4.3 Documentation | ||||
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Once a connection to a database server has been successfully established, the functions described here are used to perform SQL queries and commands.
PQexec
Submits a command to the server and waits for the result.
PGresult *PQexec(PGconn *conn, const char *command);
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 server.
If a null pointer is returned, it
should be treated like a PGRES_FATAL_ERROR result. Use
PQerrorMessage
to get more information about the error.
It is allowed to include multiple SQL commands (separated by semicolons) in
the command string. Multiple queries sent in a single PQexec
call are processed in a single transaction, unless there are explicit
BEGIN/COMMIT commands included in the query string to divide it into multiple
transactions. Note however that the returned PGresult
structure describes only the result of the last command executed from the
string. Should one of the commands fail, processing of the string stops with
it and the returned PGresult describes the error
condition.
PQexecParams
Submits a command to the server and waits for the result, with the ability to pass parameters separately from the SQL command text.
PGresult *PQexecParams(PGconn *conn, const char *command, int nParams, const Oid *paramTypes, const char * const *paramValues, const int *paramLengths, const int *paramFormats, int resultFormat);
PQexecParams
is like PQexec
, but offers additional
functionality: parameter values can be specified separately from the command
string proper, and query results can be requested in either text or binary
format. PQexecParams
is supported only in protocol 3.0 and later
connections; it will fail when using protocol 2.0.
If parameters are used, they are referred to in the command string as $1, $2, etc. nParams is the number of parameters supplied; it is the length of the arrays paramTypes[], paramValues[], paramLengths[], and paramFormats[]. (The array pointers may be NULL when nParams is zero.) paramTypes[] specifies, by OID, the data types to be assigned to the parameter symbols. If paramTypes is NULL, or any particular element in the array is zero, the server assigns a data type to the parameter symbol in the same way it would do for an untyped literal string. paramValues[] specifies the actual values of the parameters. A null pointer in this array means the corresponding parameter is null; otherwise the pointer points to a zero-terminated text string (for text format) or binary data in the format expected by the server (for binary format). paramLengths[] specifies the actual data lengths of binary-format parameters. It is ignored for null parameters and text-format parameters. The array pointer may be null when there are no binary parameters. paramFormats[] specifies whether parameters are text (put a zero in the array) or binary (put a one in the array). If the array pointer is null then all parameters are presumed to be text. resultFormat is zero to obtain results in text format, or one to obtain results in binary format. (There is not currently a provision to obtain different result columns in different formats, although that is possible in the underlying protocol.)
The primary advantage of PQexecParams
over PQexec
is that parameter values may be separated from the command string, thus
avoiding the need for tedious and error-prone quoting and escaping.
Unlike PQexec
, PQexecParams
allows at most one SQL
command in the given string. (There can be semicolons in it, but not more
than one nonempty command.) This is a limitation of the underlying protocol,
but has some usefulness as an extra defense against SQL-injection attacks.
PQexecPrepared
Sends a request to execute a prepared statement with given parameters, and waits for the result.
PGresult *PQexecPrepared(PGconn *conn, const char *stmtName, int nParams, const char * const *paramValues, const int *paramLengths, const int *paramFormats, int resultFormat);
PQexecPrepared
is like PQexecParams
, but the
command to be executed is specified by naming a previously-prepared
statement, instead of giving a query string. This feature allows commands
that will be used repeatedly to be parsed and planned just once, rather
than each time they are executed.
PQexecPrepared
is supported only in protocol 3.0 and later
connections; it will fail when using protocol 2.0.
The parameters are identical to PQexecParams
, except that the
name of a prepared statement is given instead of a query string, and the
paramTypes[] parameter is not present (it is not needed since
the prepared statement's parameter types were determined when it was created).
Presently, prepared statements for use with PQexecPrepared
must be set up by executing an SQL PREPARE command,
which is typically sent with PQexec
(though any of
libpq's query-submission functions may be used).
A lower-level interface for preparing statements may be offered in a
future release.
The PGresult structure encapsulates the result returned by the server. 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.
PQresultStatus
Returns the result status of the command.
ExecStatusType PQresultStatus(const PGresult *res);
PQresultStatus
can return one of the following values:
The string sent to the server was empty.
Successful completion of a command returning no data.
Successful completion of a command returning data (such as a SELECT or SHOW).
Copy Out (from server) data transfer started.
Copy In (to server) data transfer started.
The server's response was not understood.
A nonfatal error (a notice or warning) occurred.
A fatal error occurred.
If the result status is PGRES_TUPLES_OK, then the functions 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 may indicate a bug in the client software.
A result of status PGRES_NONFATAL_ERROR will never be
returned directly by PQexec
or other query
execution functions; results of this kind are instead passed to the notice
processor (see Section 27.9).
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 command, or an empty string if there was no error.
char *PQresultErrorMessage(const PGresult *res);
If there was an error, the returned string will include a trailing newline.
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.
PQresultErrorField
Returns an individual field of an error report.
char *PQresultErrorField(const PGresult *res, int fieldcode);
fieldcode is an error field identifier; see the symbols listed below. NULL is returned if the PGresult is not an error or warning result, or does not include the specified field. Field values will normally not include a trailing newline.
The following field codes are available:
The severity; the field contents are ERROR, FATAL, or PANIC (in an error message), or WARNING, NOTICE, DEBUG, INFO, or LOG (in a notice message), or a localized translation of one of these. Always present.
The SQLSTATE code for the error (see Appendix A). Not localizable. Always present.
The primary human-readable error message (typically one line). Always present.
Detail: an optional secondary error message carrying more detail about the problem. May run to multiple lines.
Hint: an optional suggestion what to do about the problem. This is intended to differ from detail in that it offers advice (potentially inappropriate) rather than hard facts. May run to multiple lines.
A string containing a decimal integer indicating an error cursor position as an index into the original statement string. The first character has index 1, and positions are measured in characters not bytes.
An indication of the context in which the error occurred. Presently this includes a call stack traceback of active PL functions. The trace is one entry per line, most recent first.
The file name of the source-code location where the error was reported.
The line number of the source-code location where the error was reported.
The name of the source-code function reporting the error.
The client is responsible for formatting displayed information to meet its needs; in particular it should break long lines as needed. Newline characters appearing in the error message fields should be treated as paragraph breaks, not line breaks.
Errors generated internally by libpq will have severity and primary message, but typically no other fields. Errors returned by a pre-3.0-protocol server will include severity and primary message, and sometimes a detail message, but no other fields.
Note that error fields are only available from
PGresult objects, not
PGconn objects; there is no
PQerrorField
function.
PQclear
Frees the storage associated with a PGresult.
Every command 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 command,
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 your application.
PQmakeEmptyPGresult
Constructs an empty PGresult object with the given status.
PGresult* PQmakeEmptyPGresult(PGconn *conn, ExecStatusType status);
This is libpq's internal function 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 current error message of the specified connection is copied into the PGresult.
Note that PQclear
should eventually be called on the object, just
as with a PGresult returned by libpq itself.
These functions are used to extract information from a PGresult object that represents a successful query result (that is, one that has status PGRES_TUPLES_OK). For objects with other status values they will act as though the result has zero rows and zero columns.
PQntuples
Returns the number of rows (tuples) in the query result.
int PQntuples(const PGresult *res);
PQnfields
Returns the number of columns (fields) in each row of the query result.
int PQnfields(const PGresult *res);
PQfname
Returns the column name associated with the given column number. Column numbers start at 0.
char *PQfname(const PGresult *res, int column_number);
NULL is returned if the column number is out of range.
PQfnumber
Returns the column number associated with the given column name.
int PQfnumber(const PGresult *res, const char *column_name);
-1 is returned if the given name does not match any column.
The given name is treated like an identifier in an SQL command, that is, it is downcased unless double-quoted. For example, given a query result generated from the SQL command
select 1 as FOO, 2 as "BAR";
we would have the results:
PQfname(res, 0) foo PQfname(res, 1) BAR PQfnumber(res, "FOO") 0 PQfnumber(res, "foo") 0 PQfnumber(res, "BAR") -1 PQfnumber(res, "\"BAR\"") 1
PQftable
Returns the OID of the table from which the given column was fetched. Column numbers start at 0.
Oid PQftable(const PGresult *res, int column_number);
InvalidOid is returned if the column number is out of range, or if the specified column is not a simple reference to a table column, or when using pre-3.0 protocol. You can query the system table pg_class to determine exactly which table is referenced.
The type Oid and the constant InvalidOid will be defined when you include the libpq header file. They will both be some integer type.
PQftablecol
Returns the column number (within its table) of the column making up the specified query result column. Result column numbers start at 0.
int PQftablecol(const PGresult *res, int column_number);
Zero is returned if the column number is out of range, or if the specified column is not a simple reference to a table column, or when using pre-3.0 protocol.
PQfformat
Returns the format code indicating the format of the given column. Column numbers start at 0.
int PQfformat(const PGresult *res, int column_number);
Format code zero indicates textual data representation, while format code one indicates binary representation. (Other codes are reserved for future definition.)
PQftype
Returns the data type associated with the given column number. The integer returned is the internal OID number of the type. Column numbers start at 0.
Oid PQftype(const PGresult *res, int column_number);
You can query the system table pg_type to obtain the names and properties of the various data types. The OIDs of the built-in data types are defined in the file src/include/catalog/pg_type.h in the source tree.
PQfmod
Returns the type modifier of the column associated with the given column number. Column numbers start at 0.
int PQfmod(const PGresult *res, int column_number);
The interpretation of modifier values is type-specific; they typically indicate precision or size limits. The value -1 is used to indicate "no information available". Most data types do not use modifiers, in which case the value is always -1.
PQfsize
Returns the size in bytes of the column associated with the given column number. Column numbers start at 0.
int PQfsize(const PGresult *res, int column_number);
PQfsize
returns the space allocated for this column in a database
row, in other words the size of the server's internal representation
of the data type. (Accordingly, it is not really very useful to clients.)
A negative value indicates the data type is variable-length.
PQbinaryTuples
Returns 1 if the PGresult contains binary data and 0 if it contains text data.
int PQbinaryTuples(const PGresult *res);
This function is deprecated (except for its use in connection with
COPY), because it is possible for a single
PGresult
to contain text data in some columns and binary data in others.
PQfformat
is preferred. PQbinaryTuples
returns 1 only if all columns of the result are binary (format 1).
PQgetvalue
Returns a single field value of one row of a PGresult. Row and column numbers start at 0.
char* PQgetvalue(const PGresult *res, int row_number, int column_number);
For data in text format, the value returned by PQgetvalue
is a null-terminated character string representation
of the field value. For data in binary format, the value is in the binary
representation determined by the data type's typsend
and
typreceive
functions. (The value is actually followed by
a zero byte in this case too, but that is not ordinarily useful, since
the value is likely to contain embedded nulls.)
An empty string is returned if the field value is null. See
PQgetisnull
to distinguish null values from empty-string values.
The pointer
returned by PQgetvalue
points to storage that is
part of the PGresult structure. One should not modify the data it points to,
and one must explicitly
copy the data into other storage if it is to
be used past the lifetime of the PGresult structure itself.
PQgetisnull
Tests a field for a null value. Row and column numbers start at 0.
int PQgetisnull(const PGresult *res, int row_number, int column_number);
This function returns 1 if the field is null and 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 actual length of a field value in bytes. Row and column numbers start at 0.
int PQgetlength(const PGresult *res, int row_number, int column_number);
This is the actual data length for the particular data value, that is, the
size of the object pointed to by PQgetvalue
. For text
data format this is the same as strlen()
. For binary format
this is essential information. Note that one should not rely
on PQfsize
to obtain the actual data length.
PQprint
Prints out all the rows and, optionally, the column names to the specified output stream.
void PQprint(FILE* fout, /* output stream */ const PGresult *res, const PQprintOpt *po); typedef 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; /* attributes for HTML table element */ char *caption; /* HTML table 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. Note that it assumes all the data is in text format.
These functions are used to extract information from PGresult objects that are not SELECT results.
PQcmdStatus
Returns the command status tag from the SQL command that generated the PGresult.
char * PQcmdStatus(PGresult *res);
Commonly this is just the name of the command, but it may include additional data such as the number of rows processed.
PQcmdTuples
Returns the number of rows affected by the SQL command.
char * PQcmdTuples(PGresult *res);
If the SQL command that generated the PGresult was INSERT, UPDATE, DELETE, MOVE, or FETCH, this returns a string containing the number of rows affected. If the command was anything else, it returns the empty string.
PQoidValue
Returns the OID 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);
PQoidStatus
Returns a string with the OID 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
.
It is not thread-safe.
PQescapeString
escapes a string for use within an SQL
command. This is useful when inserting data values as literal constants
in SQL commands. Certain characters (such as quotes and backslashes) must
be escaped to prevent them from being interpreted specially by the SQL parser.
PQescapeString
performs this operation.
Tip: It is especially important to do proper escaping when handling strings that were received from an untrustworthy source. Otherwise there is a security risk: you are vulnerable to "SQL injection" attacks wherein unwanted SQL commands are fed to your database.
Note that it is not necessary nor correct to do escaping when a data
value is passed as a separate parameter in PQexecParams
or
its sibling routines.
size_t PQescapeString (char *to, const char *from, size_t length);
The parameter from points to the first character of the string
that is to be escaped, and the length parameter gives the
number of characters in this string. A terminating zero byte is not
required, and should not be counted in length. (If
a terminating zero byte is found before length bytes are
processed, PQescapeString
stops at the zero; the behavior
is thus rather like strncpy
.)
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
included in the result string; they should be provided in the SQL
command that the result is inserted into.
PQescapeString
returns the number of characters written
to to, not including the terminating zero byte.
Behavior is undefined if the to and from strings overlap.
PQescapeBytea
Escapes binary data for use within an SQL command with the type
bytea. As with PQescapeString
,
this is only used when inserting data directly into an SQL command string.
unsigned char *PQescapeBytea(const unsigned char *from, size_t from_length, size_t *to_length);
Certain byte values must be escaped (but all
byte values may be escaped) when used as part
of a bytea literal in an SQL
statement. In general, to escape a byte, it is converted into the
three digit octal number equal to the octet value, and preceded by
two backslashes. The single quote (') and backslash
(\) characters have special alternative escape
sequences. See Section 8.4 for more
information. PQescapeBytea
performs this
operation, escaping only the minimally required bytes.
The from parameter points to the first byte of the string that is to be escaped, and the from_length parameter gives the number of bytes in this binary string. (A terminating zero byte is neither necessary nor counted.) The to_length parameter points to a variable that will hold the resultant escaped string length. The result string length includes the terminating zero byte of the result.
PQescapeBytea
returns an escaped version of the
from parameter binary string in memory
allocated with malloc()
. This memory must be freed
using PQfreemem
when the result is no longer needed.
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.
PQunescapeBytea
Converts an escaped string representation of binary data into binary
data --- the reverse of PQescapeBytea
.
This is needed when retrieving bytea data in text format,
but not when retrieving it in binary format.
unsigned char *PQunescapeBytea(const unsigned char *from, size_t *to_length);
The from parameter points to an escaped string
such as might be returned by PQgetvalue
when applied to a
bytea column. PQunescapeBytea
converts
this string representation into its binary representation.
It returns a pointer to a buffer allocated with
malloc()
, or null on error, and puts the size of
the buffer in to_length. The result must be
freed using PQfreemem
when it is no longer needed.
PQfreemem
Frees memory allocated by libpq.
void PQfreemem(void *ptr);
Frees memory allocated by libpq, particularly
PQescapeBytea
,
PQunescapeBytea
,
and PQnotifies
.
It is needed by Microsoft Windows, which cannot free memory across
DLLs, unless multithreaded DLLs (/MD in VC6) are used.
On other platforms, this function is the same as the standard library function free()
.