Hướng dẫn dùng psql verbosity trong PHP

Connecting to a Database

psql is a regular PostgreSQL client application. In order to connect to a database you need to know the name of your target database, the host name and port number of the server, and what user name you want to connect as. psql can be told about those parameters via command line options, namely -d, -h, -p, and -U respectively. If an argument is found that does not belong to any option it will be interpreted as the database name [or the user name, if the database name is already given]. Not all of these options are required; there are useful defaults. If you omit the host name, psql will connect via a Unix-domain socket to a server on the local host, or via TCP/IP to localhost on machines that don't have Unix-domain sockets. The default port number is determined at compile time. Since the database server uses the same default, you will not have to specify the port in most cases. The default user name is your operating-system user name, as is the default database name. Note that you cannot just connect to any database under any user name. Your database administrator should have informed you about your access rights.

When the defaults aren't quite right, you can save yourself some typing by setting the environment variables PGDATABASE, PGHOST, PGPORT and/or PGUSER to appropriate values. [For additional environment variables, see Section 34.15.] It is also convenient to have a ~/.pgpass file to avoid regularly having to type in passwords. See Section 34.16 for more information.

An alternative way to specify connection parameters is in a conninfo string or a URI, which is used instead of a database name. This mechanism give you very wide control over the connection. For example:

$ psql "service=myservice sslmode=require"
$ psql postgresql://dbmaster:5433/mydb?sslmode=require

This way you can also use LDAP for connection parameter lookup as described in Section 34.18. See Section 34.1.2 for more information on all the available connection options.

If the connection could not be made for any reason [e.g., insufficient privileges, server is not running on the targeted host, etc.], psql will return an error and terminate.

If both standard input and standard output are a terminal, then psql sets the client encoding to “auto”, which will detect the appropriate client encoding from the locale settings [LC_CTYPE environment variable on Unix systems]. If this doesn't work out as expected, the client encoding can be overridden using the environment variable PGCLIENTENCODING.

Entering SQL Commands

In normal operation, psql provides a prompt with the name of the database to which psql is currently connected, followed by the string =>. For example:

$ psql testdb
psql [14.5]
Type "help" for help.

testdb=>

At the prompt, the user can type in SQL commands. Ordinarily, input lines are sent to the server when a command-terminating semicolon is reached. An end of line does not terminate a command. Thus commands can be spread over several lines for clarity. If the command was sent and executed without error, the results of the command are displayed on the screen.

If untrusted users have access to a database that has not adopted a secure schema usage pattern, begin your session by removing publicly-writable schemas from search_path. One can add options=-csearch_path= to the connection string or issue SELECT pg_catalog.set_config['search_path', '', false] before other SQL commands. This consideration is not specific to psql; it applies to every interface for executing arbitrary SQL commands.

Whenever a command is executed, psql also polls for asynchronous notification events generated by LISTEN and NOTIFY.

While C-style block comments are passed to the server for processing and removal, SQL-standard comments are removed by psql.

Advanced Features

Variables

psql provides variable substitution features similar to common Unix command shells. Variables are simply name/value pairs, where the value can be any string of any length. The name must consist of letters [including non-Latin letters], digits, and underscores.

To set a variable, use the psql meta-command \set. For example,

testdb=> \set foo bar

sets the variable foo to the value bar. To retrieve the content of the variable, precede the name with a colon, for example:

testdb=> \echo :foo
bar

This works in both regular SQL commands and meta-commands; there is more detail in SQL Interpolation, below.

If you call \set without a second argument, the variable is set to an empty-string value. To unset [i.e., delete] a variable, use the command \unset. To show the values of all variables, call \set without any argument.

Note

The arguments of \set are subject to the same substitution rules as with other commands. Thus you can construct interesting references such as \set :foo 'something' and get “soft links” or “variable variables” of Perl or PHP fame, respectively. Unfortunately [or fortunately?], there is no way to do anything useful with these constructs. On the other hand, \set bar :foo is a perfectly valid way to copy a variable.

A number of these variables are treated specially by psql. They represent certain option settings that can be changed at run time by altering the value of the variable, or in some cases represent changeable state of psql. By convention, all specially treated variables' names consist of all upper-case ASCII letters [and possibly digits and underscores]. To ensure maximum compatibility in the future, avoid using such variable names for your own purposes.

Variables that control psql's behavior generally cannot be unset or set to invalid values. An \unset command is allowed but is interpreted as setting the variable to its default value. A \set command without a second argument is interpreted as setting the variable to on, for control variables that accept that value, and is rejected for others. Also, control variables that accept the values on and off will also accept other common spellings of Boolean values, such as true and false.

The specially treated variables are:

AUTOCOMMIT

When on [the default], each SQL command is automatically committed upon successful completion. To postpone commit in this mode, you must enter a BEGIN or START TRANSACTION SQL command. When off or unset, SQL commands are not committed until you explicitly issue COMMIT or END. The autocommit-off mode works by issuing an implicit BEGIN for you, just before any command that is not already in a transaction block and is not itself a BEGIN or other transaction-control command, nor a command that cannot be executed inside a transaction block [such as VACUUM].

Note

In autocommit-off mode, you must explicitly abandon any failed transaction by entering ABORT or ROLLBACK. Also keep in mind that if you exit the session without committing, your work will be lost.

Note

The autocommit-on mode is PostgreSQL's traditional behavior, but autocommit-off is closer to the SQL spec. If you prefer autocommit-off, you might wish to set it in the system-wide psqlrc file or your ~/.psqlrc file.

Determines which letter case to use when completing an SQL key word. If set to lower or upper, the completed word will be in lower or upper case, respectively. If set to preserve-lower or preserve-upper [the default], the completed word will be in the case of the word already entered, but words being completed without anything entered will be in lower or upper case, respectively.

The name of the database you are currently connected to. This is set every time you connect to a database [including program start-up], but can be changed or unset.

If set to all, all nonempty input lines are printed to standard output as they are read. [This does not apply to lines read interactively.] To select this behavior on program start-up, use the switch -a. If set to queries, psql prints each query to standard output as it is sent to the server. The switch to select this behavior is -e. If set to errors, then only failed queries are displayed on standard error output. The switch for this behavior is -b. If set to none [the default], then no queries are displayed.

When this variable is set to on and a backslash command queries the database, the query is first shown. This feature helps you to study PostgreSQL internals and provide similar functionality in your own programs. [To select this behavior on program start-up, use the switch -E.] If you set this variable to the value noexec, the queries are just shown but are not actually sent to the server and executed. The default value is off.

The current client character set encoding. This is set every time you connect to a database [including program start-up], and when you change the encoding with \encoding, but it can be changed or unset.

true if the last SQL query failed, false if it succeeded. See also SQLSTATE.

If this variable is set to an integer value greater than zero, the results of SELECT queries are fetched and displayed in groups of that many rows, rather than the default behavior of collecting the entire result set before display. Therefore only a limited amount of memory is used, regardless of the size of the result set. Settings of 100 to 1000 are commonly used when enabling this feature. Keep in mind that when using this feature, a query might fail after having already displayed some rows.

Tip

Although you can use any output format with this feature, the default aligned format tends to look bad because each group of FETCH_COUNT rows will be formatted separately, leading to varying column widths across the row groups. The other output formats work better.

If this variable is set to true, a table's access method details are not displayed. This is mainly useful for regression tests.

If this variable is set to true, column compression method details are not displayed. This is mainly useful for regression tests.

If this variable is set to ignorespace, lines which begin with a space are not entered into the history list. If set to a value of ignoredups, lines matching the previous history line are not entered. A value of ignoreboth combines the two options. If set to none [the default], all lines read in interactive mode are saved on the history list.

Note

This feature was shamelessly plagiarized from Bash.

The file name that will be used to store the history list. If unset, the file name is taken from the PSQL_HISTORY environment variable. If that is not set either, the default is ~/.psql_history, or %APPDATA%\postgresql\psql_history on Windows. For example, putting:

\set HISTFILE ~/.psql_history-:DBNAME

in ~/.psqlrc will cause psql to maintain a separate history for each database.

Note

This feature was shamelessly plagiarized from Bash.

The maximum number of commands to store in the command history [default 500]. If set to a negative value, no limit is applied.

Note

This feature was shamelessly plagiarized from Bash.

The database server host you are currently connected to. This is set every time you connect to a database [including program start-up], but can be changed or unset.

If set to 1 or less, sending an EOF character [usually Control+D] to an interactive session of psql will terminate the application. If set to a larger numeric value, that many consecutive EOF characters must be typed to make an interactive session terminate. If the variable is set to a non-numeric value, it is interpreted as 10. The default is 0.

Note

This feature was shamelessly plagiarized from Bash.

The value of the last affected OID, as returned from an INSERT or \lo_import command. This variable is only guaranteed to be valid until after the result of the next SQL command has been displayed. PostgreSQL servers since version 12 do not support OID system columns anymore, thus LASTOID will always be 0 following INSERT when targeting such servers.

The primary error message and associated SQLSTATE code for the most recent failed query in the current psql session, or an empty string and 00000 if no error has occurred in the current session.

When set to on, if a statement in a transaction block generates an error, the error is ignored and the transaction continues. When set to interactive, such errors are only ignored in interactive sessions, and not when reading script files. When set to off [the default], a statement in a transaction block that generates an error aborts the entire transaction. The error rollback mode works by issuing an implicit SAVEPOINT for you, just before each command that is in a transaction block, and then rolling back to the savepoint if the command fails.

By default, command processing continues after an error. When this variable is set to on, processing will instead stop immediately. In interactive mode, psql will return to the command prompt; otherwise, psql will exit, returning error code 3 to distinguish this case from fatal error conditions, which are reported using error code 1. In either case, any currently running scripts [the top-level script, if any, and any other scripts which it may have in invoked] will be terminated immediately. If the top-level command string contained multiple SQL commands, processing will stop with the current command.

The database server port to which you are currently connected. This is set every time you connect to a database [including program start-up], but can be changed or unset.

These specify what the prompts psql issues should look like. See Prompting below.

Setting this variable to on is equivalent to the command line option -q. It is probably not too useful in interactive mode.

The number of rows returned or affected by the last SQL query, or 0 if the query failed or did not report a row count.

The server's version number as a string, for example 9.6.2, 10.1 or 11beta1, and in numeric form, for example 90602 or 100001. These are set every time you connect to a database [including program start-up], but can be changed or unset.

This variable can be set to the values never, errors, or always to control whether CONTEXT fields are displayed in messages from the server. The default is errors [meaning that context will be shown in error messages, but not in notice or warning messages]. This setting has no effect when VERBOSITY is set to terse or sqlstate. [See also \errverbose, for use when you want a verbose version of the error you just got.]

Setting this variable to on is equivalent to the command line option -S.

Setting this variable to on is equivalent to the command line option -s.

The error code [see Appendix A] associated with the last SQL query's failure, or 00000 if it succeeded.

The database user you are currently connected as. This is set every time you connect to a database [including program start-up], but can be changed or unset.

This variable can be set to the values default, verbose, terse, or sqlstate to control the verbosity of error reports. [See also \errverbose, for use when you want a verbose version of the error you just got.]

These variables are set at program start-up to reflect psql's version, respectively as a verbose string, a short string [e.g., 9.6.2, 10.1, or 11beta1], and a number [e.g., 90602 or 100001]. They can be changed or unset.

SQL Interpolation

A key feature of psql variables is that you can substitute [“interpolate”] them into regular SQL statements, as well as the arguments of meta-commands. Furthermore, psql provides facilities for ensuring that variable values used as SQL literals and identifiers are properly quoted. The syntax for interpolating a value without any quoting is to prepend the variable name with a colon [:]. For example,

testdb=> \set foo 'my_table'
testdb=> SELECT * FROM :foo;

would query the table my_table. Note that this may be unsafe: the value of the variable is copied literally, so it can contain unbalanced quotes, or even backslash commands. You must make sure that it makes sense where you put it.

When a value is to be used as an SQL literal or identifier, it is safest to arrange for it to be quoted. To quote the value of a variable as an SQL literal, write a colon followed by the variable name in single quotes. To quote the value as an SQL identifier, write a colon followed by the variable name in double quotes. These constructs deal correctly with quotes and other special characters embedded within the variable value. The previous example would be more safely written this way:

testdb=> \set foo 'my_table'
testdb=> SELECT * FROM :"foo";

Variable interpolation will not be performed within quoted SQL literals and identifiers. Therefore, a construction such as ':foo' doesn't work to produce a quoted literal from a variable's value [and it would be unsafe if it did work, since it wouldn't correctly handle quotes embedded in the value].

One example use of this mechanism is to copy the contents of a file into a table column. First load the file into a variable and then interpolate the variable's value as a quoted string:

testdb=> \set content `cat my_file.txt`
testdb=> INSERT INTO my_table VALUES [:'content'];

[Note that this still won't work if my_file.txt contains NUL bytes. psql does not support embedded NUL bytes in variable values.]

Since colons can legally appear in SQL commands, an apparent attempt at interpolation [that is, :name, :'name', or :"name"] is not replaced unless the named variable is currently set. In any case, you can escape a colon with a backslash to protect it from substitution.

The :{?name} special syntax returns TRUE or FALSE depending on whether the variable exists or not, and is thus always substituted, unless the colon is backslash-escaped.

The colon syntax for variables is standard SQL for embedded query languages, such as ECPG. The colon syntaxes for array slices and type casts are PostgreSQL extensions, which can sometimes conflict with the standard usage. The colon-quote syntax for escaping a variable's value as an SQL literal or identifier is a psql extension.

Prompting

The prompts psql issues can be customized to your preference. The three variables PROMPT1, PROMPT2, and PROMPT3 contain strings and special escape sequences that describe the appearance of the prompt. Prompt 1 is the normal prompt that is issued when psql requests a new command. Prompt 2 is issued when more input is expected during command entry, for example because the command was not terminated with a semicolon or a quote was not closed. Prompt 3 is issued when you are running an SQL COPY FROM STDIN command and you need to type in a row value on the terminal.

The value of the selected prompt variable is printed literally, except where a percent sign [%] is encountered. Depending on the next character, certain other text is substituted instead. Defined substitutions are:

%M

The full host name [with domain name] of the database server, or [local] if the connection is over a Unix domain socket, or [local:/dir/name], if the Unix domain socket is not at the compiled in default location.

The host name of the database server, truncated at the first dot, or [local] if the connection is over a Unix domain socket.

The port number at which the database server is listening.

The database session user name. [The expansion of this value might change during a database session as the result of the command SET SESSION AUTHORIZATION.]

The name of the current database.

Like %/, but the output is ~ [tilde] if the database is your default database.

If the session user is a database superuser, then a #, otherwise a >. [The expansion of this value might change during a database session as the result of the command SET SESSION AUTHORIZATION.]

The process ID of the backend currently connected to.

In prompt 1 normally =, but @ if the session is in an inactive branch of a conditional block, or ^ if in single-line mode, or ! if the session is disconnected from the database [which can happen if \connect fails]. In prompt 2 %R is replaced by a character that depends on why psql expects more input: - if the command simply wasn't terminated yet, but * if there is an unfinished /* ... */ comment, a single quote if there is an unfinished quoted string, a double quote if there is an unfinished quoted identifier, a dollar sign if there is an unfinished dollar-quoted string, or [ if there is an unmatched left parenthesis. In prompt 3 %R doesn't produce anything.

Transaction status: an empty string when not in a transaction block, or * when in a transaction block, or ! when in a failed transaction block, or ? when the transaction state is indeterminate [for example, because there is no connection].

The line number inside the current statement, starting from 1.

The character with the indicated octal code is substituted.

The value of the psql variable name. See Variables, above, for details.

The output of command, similar to ordinary “back-tick” substitution.

Prompts can contain terminal control characters which, for example, change the color, background, or style of the prompt text, or change the title of the terminal window. In order for the line editing features of Readline to work properly, these non-printing control characters must be designated as invisible by surrounding them with %[ and %]. Multiple pairs of these can occur within the prompt. For example:

testdb=> \set PROMPT1 '%[%033[1;33;40m%]%n@%/%R%[%033[0m%]%# '

results in a boldfaced [1;] yellow-on-black [33;40] prompt on VT100-compatible, color-capable terminals.

Whitespace of the same width as the most recent output of PROMPT1. This can be used as a PROMPT2 setting, so that multi-line statements are aligned with the first line, but there is no visible secondary prompt.

To insert a percent sign into your prompt, write %%. The default prompts are '%/%R%x%# ' for prompts 1 and 2, and '>> ' for prompt 3.

Note

This feature was shamelessly plagiarized from tcsh.

Command-Line Editing

psql supports the Readline library for convenient line editing and retrieval. The command history is automatically saved when psql exits and is reloaded when psql starts up. Tab-completion is also supported, although the completion logic makes no claim to be an SQL parser. The queries generated by tab-completion can also interfere with other SQL commands, e.g., SET TRANSACTION ISOLATION LEVEL. If for some reason you do not like the tab completion, you can turn it off by putting this in a file named .inputrc in your home directory:

$if psql
set disable-completion on
$endif

[This is not a psql but a Readline feature. Read its documentation for further details.]