43.5. Basic Statements
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## 43.5. Basic Statements #

In this section and the following ones, we describe all the statement types that are explicitly understood by PL/pgSQL. Anything not recognized as one of these statement types is presumed to be an SQL command and is sent to the main database engine to execute, as described in Section 43.5.2.

### 43.5.1. Assignment #

An assignment of a value to a PL/pgSQL variable is written as:

    variable { := | = } expression;

As explained previously, the expression in such a statement is evaluated by means of an SQL `SELECT` command sent to the main database engine. The expression must yield a single value (possibly a row value, if the variable is a row or record variable). The target variable can be a simple variable (optionally qualified with a block name), a field of a row or record target, or an element or slice of an array target. Equal (`=`) can be used instead of PL/SQL-compliant `:=`.

If the expression's result data type doesn't match the variable's data type, the value will be coerced as though by an assignment cast (see Section 10.4). If no assignment cast is known for the pair of data types involved, the PL/pgSQL interpreter will attempt to convert the result value textually, that is by applying the result type's output function followed by the variable type's input function. Note that this could result in run-time errors generated by the input function, if the string form of the result value is not acceptable to the input function.

Examples:

    tax := subtotal * 0.06;
    my_record.user_id := 20;
    my_array[j] := 20;
    my_array[1:3] := array[1,2,3];
    complex_array[n].realpart = 12.3;

### 43.5.2. Executing SQL Commands #

In general, any SQL command that does not return rows can be executed within a PL/pgSQL function just by writing the command. For example, you could create and fill a table by writing

    CREATE TABLE mytable (id int primary key, data text);
    INSERT INTO mytable VALUES (1,'one'), (2,'two');

If the command does return rows (for example `SELECT`, or `INSERT`/`UPDATE`/`DELETE` with `RETURNING`), there are two ways to proceed. When the command will return at most one row, or you only care about the first row of output, write the command as usual but add an `INTO` clause to capture the output, as described in Section 43.5.3. To process all of the output rows, write the command as the data source for a `FOR` loop, as described in Section 43.6.6.

Usually it is not sufficient just to execute statically-defined SQL commands. Typically you'll want a command to use varying data values, or even to vary in more fundamental ways such as by using different table names at different times. Again, there are two ways to proceed depending on the situation.

PL/pgSQL variable values can be automatically inserted into optimizable SQL commands, which are `SELECT`, `INSERT`, `UPDATE`, `DELETE`, `MERGE`, and certain utility commands that incorporate one of these, such as `EXPLAIN` and `CREATE TABLE ... AS SELECT`. In these commands, any PL/pgSQL variable name appearing in the command text is replaced by a query parameter, and then the current value of the variable is provided as the parameter value at run time. This is exactly like the processing described earlier for expressions; for details see Section 43.11.1.

When executing an optimizable SQL command in this way, PL/pgSQL may cache and re-use the execution plan for the command, as discussed in Section 43.11.2.

Non-optimizable SQL commands (also called utility commands) are not capable of accepting query parameters. So automatic substitution of PL/pgSQL variables does not work in such commands. To include non-constant text in a utility command executed from PL/pgSQL, you must build the utility command as a string and then `EXECUTE` it, as discussed in Section 43.5.4.

`EXECUTE` must also be used if you want to modify the command in some other way than supplying a data value, for example by changing a table name.

Sometimes it is useful to evaluate an expression or `SELECT` query but discard the result, for example when calling a function that has side-effects but no useful result value. To do this in PL/pgSQL, use the `PERFORM` statement:

    PERFORM query;

This executes `query` and discards the result. Write the `query` the same way you would write an SQL `SELECT` command, but replace the initial keyword `SELECT` with `PERFORM`. For `WITH` queries, use `PERFORM` and then place the query in parentheses. (In this case, the query can only return one row.) PL/pgSQL variables will be substituted into the query just as described above, and the plan is cached in the same way. Also, the special variable `FOUND` is set to true if the query produced at least one row, or false if it produced no rows (see Section 43.5.5).

### Note

One might expect that writing `SELECT` directly would accomplish this result, but at present the only accepted way to do it is `PERFORM`. An SQL command that can return rows, such as `SELECT`, will be rejected as an error unless it has an `INTO` clause as discussed in the next section.

An example:

    PERFORM create_mv('cs_session_page_requests_mv', my_query);

### 43.5.3. Executing a Command with a Single-Row Result #

The result of an SQL command yielding a single row (possibly of multiple columns) can be assigned to a record variable, row-type variable, or list of scalar variables. This is done by writing the base SQL command and adding an `INTO` clause. For example,

    SELECT select_expressions INTO [STRICT] target FROM ...;
    INSERT ... RETURNING expressions INTO [STRICT] target;
    UPDATE ... RETURNING expressions INTO [STRICT] target;
    DELETE ... RETURNING expressions INTO [STRICT] target;

where `target` can be a record variable, a row variable, or a comma-separated list of simple variables and record/row fields. PL/pgSQL variables will be substituted into the rest of the command (that is, everything but the `INTO` clause) just as described above, and the plan is cached in the same way. This works for `SELECT`, `INSERT`/`UPDATE`/`DELETE` with `RETURNING`, and certain utility commands that return row sets, such as `EXPLAIN`. Except for the `INTO` clause, the SQL command is the same as it would be written outside PL/pgSQL.

### Tip

Note that this interpretation of `SELECT` with `INTO` is quite different from PostgreSQL's regular `SELECT INTO` command, wherein the `INTO` target is a newly created table. If you want to create a table from a `SELECT` result inside a PL/pgSQL function, use the syntax `CREATE TABLE ... AS SELECT`.

If a row variable or a variable list is used as target, the command's result columns must exactly match the structure of the target as to number and data types, or else a run-time error occurs. When a record variable is the target, it automatically configures itself to the row type of the command's result columns.

The `INTO` clause can appear almost anywhere in the SQL command. Customarily it is written either just before or just after the list of `select_expressions` in a `SELECT` command, or at the end of the command for other command types. It is recommended that you follow this convention in case the PL/pgSQL parser becomes stricter in future versions.

If `STRICT` is not specified in the `INTO` clause, then `target` will be set to the first row returned by the command, or to nulls if the command returned no rows. (Note that “the first row” is not well-defined unless you've used `ORDER BY`.) Any result rows after the first row are discarded. You can check the special `FOUND` variable (see Section 43.5.5) to determine whether a row was returned:

    SELECT * INTO myrec FROM emp WHERE empname = myname;
    IF NOT FOUND THEN
        RAISE EXCEPTION 'employee % not found', myname;
    END IF;

If the `STRICT` option is specified, the command must return exactly one row or a run-time error will be reported, either `NO_DATA_FOUND` (no rows) or `TOO_MANY_ROWS` (more than one row). You can use an exception block if you wish to catch the error, for example:

    BEGIN
        SELECT * INTO STRICT myrec FROM emp WHERE empname = myname;
        EXCEPTION
            WHEN NO_DATA_FOUND THEN
                RAISE EXCEPTION 'employee % not found', myname;
            WHEN TOO_MANY_ROWS THEN
                RAISE EXCEPTION 'employee % not unique', myname;
    END;

Successful execution of a command with `STRICT` always sets `FOUND` to true.

For `INSERT`/`UPDATE`/`DELETE` with `RETURNING`, PL/pgSQL reports an error for more than one returned row, even when `STRICT` is not specified. This is because there is no option such as `ORDER BY` with which to determine which affected row should be returned.

If `print_strict_params` is enabled for the function, then when an error is thrown because the requirements of `STRICT` are not met, the `DETAIL` part of the error message will include information about the parameters passed to the command. You can change the `print_strict_params` setting for all functions by setting `plpgsql.print_strict_params`, though only subsequent function compilations will be affected. You can also enable it on a per-function basis by using a compiler option, for example:

    CREATE FUNCTION get_userid(username text) RETURNS int
    AS $$
    #print_strict_params on
    DECLARE
    userid int;
    BEGIN
        SELECT users.userid INTO STRICT userid
            FROM users WHERE users.username = get_userid.username;
        RETURN userid;
    END;
    $$ LANGUAGE plpgsql;

On failure, this function might produce an error message such as

    ERROR:  query returned no rows
    DETAIL:  parameters: username = 'nosuchuser'
    CONTEXT:  PL/pgSQL function get_userid(text) line 6 at SQL statement

### Note

The `STRICT` option matches the behavior of Oracle PL/SQL's `SELECT INTO` and related statements.

### 43.5.4. Executing Dynamic Commands #

Oftentimes you will want to generate dynamic commands inside your PL/pgSQL functions, that is, commands that will involve different tables or different data types each time they are executed. PL/pgSQL's normal attempts to cache plans for commands (as discussed in Section 43.11.2) will not work in such scenarios. To handle this sort of problem, the `EXECUTE` statement is provided:

    EXECUTE command-string [ INTO [STRICT] target ] [ USING expression [, ... ] ];

where `command-string` is an expression yielding a string (of type `text`) containing the command to be executed. The optional `target` is a record variable, a row variable, or a comma-separated list of simple variables and record/row fields, into which the results of the command will be stored. The optional `USING` expressions supply values to be inserted into the command.

No substitution of PL/pgSQL variables is done on the computed command string. Any required variable values must be inserted in the command string as it is constructed; or you can use parameters as described below.

Also, there is no plan caching for commands executed via `EXECUTE`. Instead, the command is always planned each time the statement is run. Thus the command string can be dynamically created within the function to perform actions on different tables and columns.

The `INTO` clause specifies where the results of an SQL command returning rows should be assigned. If a row variable or variable list is provided, it must exactly match the structure of the command's results; if a record variable is provided, it will configure itself to match the result structure automatically. If multiple rows are returned, only the first will be assigned to the `INTO` variable(s). If no rows are returned, NULL is assigned to the `INTO` variable(s). If no `INTO` clause is specified, the command results are discarded.

If the `STRICT` option is given, an error is reported unless the command produces exactly one row.

The command string can use parameter values, which are referenced in the command as `$1`, `$2`, etc. These symbols refer to values supplied in the `USING` clause. This method is often preferable to inserting data values into the command string as text: it avoids run-time overhead of converting the values to text and back, and it is much less prone to SQL-injection attacks since there is no need for quoting or escaping. An example is:

    EXECUTE 'SELECT count(*) FROM mytable WHERE inserted_by = $1 AND inserted <= $2'
       INTO c
       USING checked_user, checked_date;

Note that parameter symbols can only be used for data values — if you want to use dynamically determined table or column names, you must insert them into the command string textually. For example, if the preceding query needed to be done against a dynamically selected table, you could do this:

    EXECUTE 'SELECT count(*) FROM '
        || quote_ident(tabname)
        || ' WHERE inserted_by = $1 AND inserted <= $2'
       INTO c
       USING checked_user, checked_date;

A cleaner approach is to use `format()`'s `%I` specification to insert table or column names with automatic quoting:

    EXECUTE format('SELECT count(*) FROM %I '
       'WHERE inserted_by = $1 AND inserted <= $2', tabname)
       INTO c
       USING checked_user, checked_date;

(This example relies on the SQL rule that string literals separated by a newline are implicitly concatenated.)

Another restriction on parameter symbols is that they only work in optimizable SQL commands (`SELECT`, `INSERT`, `UPDATE`, `DELETE`, `MERGE`, and certain commands containing one of these). In other statement types (generically called utility statements), you must insert values textually even if they are just data values.

An `EXECUTE` with a simple constant command string and some `USING` parameters, as in the first example above, is functionally equivalent to just writing the command directly in PL/pgSQL and allowing replacement of PL/pgSQL variables to happen automatically. The important difference is that `EXECUTE` will re-plan the command on each execution, generating a plan that is specific to the current parameter values; whereas PL/pgSQL may otherwise create a generic plan and cache it for re-use. In situations where the best plan depends strongly on the parameter values, it can be helpful to use `EXECUTE` to positively ensure that a generic plan is not selected.

`SELECT INTO` is not currently supported within `EXECUTE`; instead, execute a plain `SELECT` command and specify `INTO` as part of the `EXECUTE` itself.

### Note

The PL/pgSQL `EXECUTE` statement is not related to the `EXECUTE` SQL statement supported by the PostgreSQL server. The server's `EXECUTE` statement cannot be used directly within PL/pgSQL functions (and is not needed).

Example 43.1. Quoting Values in Dynamic Queries

When working with dynamic commands you will often have to handle escaping of single quotes. The recommended method for quoting fixed text in your function body is dollar quoting. (If you have legacy code that does not use dollar quoting, please refer to the overview in Section 43.12.1, which can save you some effort when translating said code to a more reasonable scheme.)

Dynamic values require careful handling since they might contain quote characters. An example using `format()` (this assumes that you are dollar quoting the function body so quote marks need not be doubled):

    EXECUTE format('UPDATE tbl SET %I = $1 '
       'WHERE key = $2', colname) USING newvalue, keyvalue;

It is also possible to call the quoting functions directly:

    EXECUTE 'UPDATE tbl SET '
            || quote_ident(colname)
            || ' = '
            || quote_literal(newvalue)
            || ' WHERE key = '
            || quote_literal(keyvalue);

This example demonstrates the use of the `quote_ident` and `quote_literal` functions (see Section 9.4). For safety, expressions containing column or table identifiers should be passed through `quote_ident` before insertion in a dynamic query. Expressions containing values that should be literal strings in the constructed command should be passed through `quote_literal`. These functions take the appropriate steps to return the input text enclosed in double or single quotes respectively, with any embedded special characters properly escaped.

Because `quote_literal` is labeled `STRICT`, it will always return null when called with a null argument. In the above example, if `newvalue` or `keyvalue` were null, the entire dynamic query string would become null, leading to an error from `EXECUTE`. You can avoid this problem by using the `quote_nullable` function, which works the same as `quote_literal` except that when called with a null argument it returns the string `NULL`. For example,

    EXECUTE 'UPDATE tbl SET '
            || quote_ident(colname)
            || ' = '
            || quote_nullable(newvalue)
            || ' WHERE key = '
            || quote_nullable(keyvalue);

If you are dealing with values that might be null, you should usually use `quote_nullable` in place of `quote_literal`.

As always, care must be taken to ensure that null values in a query do not deliver unintended results. For example the `WHERE` clause

    'WHERE key = ' || quote_nullable(keyvalue)

will never succeed if `keyvalue` is null, because the result of using the equality operator `=` with a null operand is always null. If you wish null to work like an ordinary key value, you would need to rewrite the above as

    'WHERE key IS NOT DISTINCT FROM ' || quote_nullable(keyvalue)

(At present, `IS NOT DISTINCT FROM` is handled much less efficiently than `=`, so don't do this unless you must. See Section 9.2 for more information on nulls and `IS DISTINCT`.)

Note that dollar quoting is only useful for quoting fixed text. It would be a very bad idea to try to write this example as:

    EXECUTE 'UPDATE tbl SET '
            || quote_ident(colname)
            || ' = $$'
            || newvalue
            || '$$ WHERE key = '
            || quote_literal(keyvalue);

because it would break if the contents of `newvalue` happened to contain `$$`. The same objection would apply to any other dollar-quoting delimiter you might pick. So, to safely quote text that is not known in advance, you must use `quote_literal`, `quote_nullable`, or `quote_ident`, as appropriate.

Dynamic SQL statements can also be safely constructed using the `format` function (see Section 9.4.1). For example:

    EXECUTE format('UPDATE tbl SET %I = %L '
       'WHERE key = %L', colname, newvalue, keyvalue);

`%I` is equivalent to `quote_ident`, and `%L` is equivalent to `quote_nullable`. The `format` function can be used in conjunction with the `USING` clause:

    EXECUTE format('UPDATE tbl SET %I = $1 WHERE key = $2', colname)
       USING newvalue, keyvalue;

This form is better because the variables are handled in their native data type format, rather than unconditionally converting them to text and quoting them via `%L`. It is also more efficient.

A much larger example of a dynamic command and `EXECUTE` can be seen in Example 43.10, which builds and executes a `CREATE FUNCTION` command to define a new function.

### 43.5.5. Obtaining the Result Status #

There are several ways to determine the effect of a command. The first method is to use the `GET DIAGNOSTICS` command, which has the form:

    GET [ CURRENT ] DIAGNOSTICS variable { = | := } item [ , ... ];

This command allows retrieval of system status indicators. `CURRENT` is a noise word (but see also `GET STACKED DIAGNOSTICS` in Section 43.6.8.1). Each `item` is a key word identifying a status value to be assigned to the specified `variable` (which should be of the right data type to receive it). The currently available status items are shown in Table 43.1. Colon-equal (`:=`) can be used instead of the SQL-standard `=` token. An example:

    GET DIAGNOSTICS integer_var = ROW_COUNT;

Table 43.1. Available Diagnostics Items

Name | Type | Description
--- | --- | ---
`ROW_COUNT` | `bigint` | the number of rows processed by the most recent SQL command
`PG_CONTEXT` | `text` | line(s) of text describing the current call stack (see Section 43.6.9)
`PG_ROUTINE_OID` | `oid` | OID of the current function

The second method to determine the effects of a command is to check the special variable named `FOUND`, which is of type `boolean`. `FOUND` starts out false within each PL/pgSQL function call. It is set by each of the following types of statements:

- A `SELECT INTO` statement sets `FOUND` true if a row is assigned, false if no row is returned.

- A `PERFORM` statement sets `FOUND` true if it produces (and discards) one or more rows, false if no row is produced.

- `UPDATE`, `INSERT`, `DELETE`, and `MERGE` statements set `FOUND` true if at least one row is affected, false if no row is affected.

- A `FETCH` statement sets `FOUND` true if it returns a row, false if no row is returned.

- A `MOVE` statement sets `FOUND` true if it successfully repositions the cursor, false otherwise.

- A `FOR` or `FOREACH` statement sets `FOUND` true if it iterates one or more times, else false. `FOUND` is set this way when the loop exits; inside the execution of the loop, `FOUND` is not modified by the loop statement, although it might be changed by the execution of other statements within the loop body.

- `RETURN QUERY` and `RETURN QUERY EXECUTE` statements set `FOUND` true if the query returns at least one row, false if no row is returned.

Other PL/pgSQL statements do not change the state of `FOUND`. Note in particular that `EXECUTE` changes the output of `GET DIAGNOSTICS`, but does not change `FOUND`.

`FOUND` is a local variable within each PL/pgSQL function; any changes to it affect only the current function.

### 43.5.6. Doing Nothing At All #

Sometimes a placeholder statement that does nothing is useful. For example, it can indicate that one arm of an if/then/else chain is deliberately empty. For this purpose, use the `NULL` statement:

    NULL;

For example, the following two fragments of code are equivalent:

    BEGIN
        y := x / 0;
    EXCEPTION
        WHEN division_by_zero THEN
            NULL;  -- ignore the error
    END;

    BEGIN
        y := x / 0;
    EXCEPTION
        WHEN division_by_zero THEN  -- ignore the error
    END;

Which is preferable is a matter of taste.

### Note

In Oracle's PL/SQL, empty statement lists are not allowed, and so `NULL` statements are required for situations such as this. PL/pgSQL allows you to just write nothing, instead.

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