Postgres-XC 1.0.3 Documentation | ||||
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Note: The following description applies both to Postgres-XC and PostgreSQL if not described explicitly.
The COPY command in Postgres-XC has options to read from or write to the network connection used by libpq. The functions described in this section allow applications to take advantage of this capability by supplying or consuming copied data.
The overall process is that the application first issues the SQL
COPY command via PQexec
or one
of the equivalent functions. The response to this (if there is no
error in the command) will be a PGresult object bearing
a status code of PGRES_COPY_OUT or
PGRES_COPY_IN (depending on the specified copy
direction). The application should then use the functions of this
section to receive or transmit data rows. When the data transfer is
complete, another PGresult object is returned to indicate
success or failure of the transfer. Its status will be
PGRES_COMMAND_OK for success or
PGRES_FATAL_ERROR if some problem was encountered.
At this point further SQL commands can be issued via
PQexec
. (It is not possible to execute other SQL
commands using the same connection while the COPY
operation is in progress.)
If a COPY command is issued via
PQexec
in a string that could contain additional
commands, the application must continue fetching results via
PQgetResult
after completing the COPY
sequence. Only when PQgetResult
returns
NULL is it certain that the PQexec
command string is done and it is safe to issue more commands.
The functions of this section should be executed only after obtaining
a result status of PGRES_COPY_OUT or
PGRES_COPY_IN from PQexec
or
PQgetResult
.
A PGresult object bearing one of these status values carries some additional data about the COPY operation that is starting. This additional data is available using functions that are also used in connection with query results:
PQnfields
Returns the number of columns (fields) to be copied.
PQbinaryTuples
0 indicates the overall copy format is textual (rows separated by newlines, columns separated by separator characters, etc). 1 indicates the overall copy format is binary. See COPY for more information.
PQfformat
Returns the format code (0 for text, 1 for binary) associated with each column of the copy operation. The per-column format codes will always be zero when the overall copy format is textual, but the binary format can support both text and binary columns. (However, as of the current implementation of COPY, only binary columns appear in a binary copy; so the per-column formats always match the overall format at present.)
Note: These additional data values are only available when using protocol 3.0. When using protocol 2.0, all these functions will return 0.
Note: The following description applies both to Postgres-XC and PostgreSQL if not described explicitly.
These functions are used to send data during COPY FROM STDIN. They will fail if called when the connection is not in COPY_IN state.
PQputCopyData
Sends data to the server during COPY_IN state.
int PQputCopyData(PGconn *conn, const char *buffer, int nbytes);
Transmits the COPY data in the specified
buffer, of length nbytes, to the server.
The result is 1 if the data was sent, zero if it was not sent
because the attempt would block (this case is only possible if the
connection is in nonblocking mode), or -1 if an error occurred.
(Use PQerrorMessage
to retrieve details if
the return value is -1. If the value is zero, wait for write-ready
and try again.)
The application can divide the COPY data stream into buffer loads of any convenient size. Buffer-load boundaries have no semantic significance when sending. The contents of the data stream must match the data format expected by the COPY command; see COPY for details.
PQputCopyEnd
Sends end-of-data indication to the server during COPY_IN state.
int PQputCopyEnd(PGconn *conn, const char *errormsg);
Ends the COPY_IN operation successfully if errormsg is NULL. If errormsg is not NULL then the COPY is forced to fail, with the string pointed to by errormsg used as the error message. (One should not assume that this exact error message will come back from the server, however, as the server might have already failed the COPY for its own reasons. Also note that the option to force failure does not work when using pre-3.0-protocol connections.)
The result is 1 if the termination data was sent, zero if it was
not sent because the attempt would block (this case is only possible
if the connection is in nonblocking mode), or -1 if an error
occurred. (Use PQerrorMessage
to retrieve
details if the return value is -1. If the value is zero, wait for
write-ready and try again.)
After successfully calling PQputCopyEnd
, call
PQgetResult
to obtain the final result status of the
COPY command. One can wait for this result to be
available in the usual way. Then return to normal operation.
Note: The following description applies both to Postgres-XC and PostgreSQL if not described explicitly.
These functions are used to receive data during COPY TO STDOUT. They will fail if called when the connection is not in COPY_OUT state.
PQgetCopyData
Receives data from the server during COPY_OUT state.
int PQgetCopyData(PGconn *conn, char **buffer, int async);
Attempts to obtain another row of data from the server during a
COPY. Data is always returned one data row at
a time; if only a partial row is available, it is not returned.
Successful return of a data row involves allocating a chunk of
memory to hold the data. The buffer parameter must
be non-NULL. *buffer is set to
point to the allocated memory, or to NULL in cases
where no buffer is returned. A non-NULL result
buffer should be freed using PQfreemem
when no longer
needed.
When a row is successfully returned, the return value is the number
of data bytes in the row (this will always be greater than zero).
The returned string is always null-terminated, though this is
probably only useful for textual COPY. A result
of zero indicates that the COPY is still in
progress, but no row is yet available (this is only possible when
async is true). A result of -1 indicates that the
COPY is done. A result of -2 indicates that an
error occurred (consult PQerrorMessage
for the reason).
When async is true (not zero),
PQgetCopyData
will not block waiting for input; it
will return zero if the COPY is still in progress
but no complete row is available. (In this case wait for read-ready
and then call PQconsumeInput
before calling
PQgetCopyData
again.) When async is
false (zero), PQgetCopyData
will block until data is
available or the operation completes.
After PQgetCopyData
returns -1, call
PQgetResult
to obtain the final result status of the
COPY command. One can wait for this result to be
available in the usual way. Then return to normal operation.
Note: The following description applies both to Postgres-XC and PostgreSQL if not described explicitly.
These functions represent older methods of handling COPY. Although they still work, they are deprecated due to poor error handling, inconvenient methods of detecting end-of-data, and lack of support for binary or nonblocking transfers.
PQgetline
Reads a newline-terminated line of characters (transmitted by the server) into a buffer string of size length.
int PQgetline(PGconn *conn, char *buffer, int length);
This function copies up to length-1 characters into
the buffer and converts the terminating newline into a zero byte.
PQgetline
returns EOF at the
end of input, 0 if the entire line has been read, and 1 if the
buffer is full but the terminating newline has not yet been read.
Note that the application must check to see if a new line consists of the two characters \., which indicates that the server has finished sending the results of the COPY command. If the application might receive lines that are more than length-1 characters long, care is needed to be sure it recognizes the \. line correctly (and does not, for example, mistake the end of a long data line for a terminator line).
PQgetlineAsync
Reads a row of COPY data (transmitted by the server) into a buffer without blocking.
int PQgetlineAsync(PGconn *conn, char *buffer, int bufsize);
This function is similar to PQgetline
, but it can be used
by applications
that must read COPY data asynchronously, that is, without blocking.
Having issued the COPY command and gotten a PGRES_COPY_OUT
response, the
application should call PQconsumeInput
and
PQgetlineAsync
until the
end-of-data signal is detected.
Unlike PQgetline
, this function takes
responsibility for detecting end-of-data.
On each call, PQgetlineAsync
will return data if a
complete data row is available in libpq's input buffer.
Otherwise, no data is returned until the rest of the row arrives.
The function returns -1 if the end-of-copy-data marker has been recognized,
or 0 if no data is available, or a positive number giving the number of
bytes of data returned. If -1 is returned, the caller must next call
PQendcopy
, and then return to normal processing.
The data returned will not extend beyond a data-row boundary. If possible a whole row will be returned at one time. But if the buffer offered by the caller is too small to hold a row sent by the server, then a partial data row will be returned. With textual data this can be detected by testing whether the last returned byte is \n or not. (In a binary COPY, actual parsing of the COPY data format will be needed to make the equivalent determination.) The returned string is not null-terminated. (If you want to add a terminating null, be sure to pass a bufsize one smaller than the room actually available.)
PQputline
Sends a null-terminated string to the server. Returns 0 if OK and EOF if unable to send the string.
int PQputline(PGconn *conn, const char *string);
The COPY data stream sent by a series of calls
to PQputline
has the same format as that
returned by PQgetlineAsync
, except that
applications are not obliged to send exactly one data row per
PQputline
call; it is okay to send a partial
line or multiple lines per call.
Note: Before PostgreSQL protocol 3.0, it was necessary for the application to explicitly send the two characters \. as a final line to indicate to the server that it had finished sending COPY data. While this still works, it is deprecated and the special meaning of \. can be expected to be removed in a future release. It is sufficient to call
PQendcopy
after having sent the actual data.
PQputnbytes
Sends a non-null-terminated string to the server. Returns 0 if OK and EOF if unable to send the string.
int PQputnbytes(PGconn *conn, const char *buffer, int nbytes);
This is exactly like PQputline
, except that the data
buffer need not be null-terminated since the number of bytes to send is
specified directly. Use this procedure when sending binary data.
PQendcopy
Synchronizes with the server.
int PQendcopy(PGconn *conn);
This function waits until the server has finished the copying.
It should either be issued when the last string has been sent
to the server using PQputline
or when the
last string has been received from the server using
PGgetline
. It must be issued or the server
will get "out of sync" with the client. Upon return
from this function, the server is ready to receive the next SQL
command. The return value is 0 on successful completion,
nonzero otherwise. (Use PQerrorMessage
to
retrieve details if the return value is nonzero.)
When using PQgetResult
, the application should
respond to a PGRES_COPY_OUT result by executing
PQgetline
repeatedly, followed by
PQendcopy
after the terminator line is seen.
It should then return to the PQgetResult
loop
until PQgetResult
returns a null pointer.
Similarly a PGRES_COPY_IN result is processed
by a series of PQputline
calls followed by
PQendcopy
, then return to the
PQgetResult
loop. This arrangement will
ensure that a COPY command embedded in a series
of SQL commands will be executed correctly.
Older applications are likely to submit a COPY
via PQexec
and assume that the transaction
is done after PQendcopy
. This will work
correctly only if the COPY is the only
SQL command in the command string.