Functions Associated with the COPY Command
PostgreSQL 9.5.8 Documentation | |||
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The COPY command in PostgreSQL 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.
31.9.1. Functions for Sending COPY Data
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 queued, zero if it was not queued because of full buffers (this will only happen 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 message was sent; or in nonblocking mode, this may only indicate that the termination message was successfully queued. (In nonblocking mode, to be certain that the data has been sent, you should next wait for write-ready and call
PQflush
, repeating until it returns zero.) Zero indicates that the function could not queue the termination message because of full buffers; this will only happen in nonblocking mode. (In this case, wait for write-ready and try thePQputCopyEnd
call again.) If a hard error occurs, -1 is returned; you can usePQerrorMessage
to retrieve details.After successfully calling
PQputCopyEnd
, callPQgetResult
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.
31.9.2. Functions for Receiving COPY Data
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 callPQconsumeInput
before callingPQgetCopyData
again.) When async is false (zero),PQgetCopyData
will block until data is available or the operation completes.After
PQgetCopyData
returns -1, callPQgetResult
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.
31.9.3. Obsolete Functions for COPY
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 callPQconsumeInput
andPQgetlineAsync
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 callPQendcopy
, 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 byPQgetlineAsync
, except that applications are not obliged to send exactly one data row perPQputline
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 usingPGgetline
. 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. (UsePQerrorMessage
to retrieve details if the return value is nonzero.)When using
PQgetResult
, the application should respond to a PGRES_COPY_OUT result by executingPQgetline
repeatedly, followed byPQendcopy
after the terminator line is seen. It should then return to thePQgetResult
loop untilPQgetResult
returns a null pointer. Similarly a PGRES_COPY_IN result is processed by a series ofPQputline
calls followed byPQendcopy
, then return to thePQgetResult
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 afterPQendcopy
. This will work correctly only if the COPY is the only SQL command in the command string.