PostgreSQL 8.4.2 Documentation
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# 18.12. Version and Platform Compatibility

## 18.12.1. Previous PostgreSQL Versions

`add_missing_from` (`boolean`)

  When on, tables that are referenced by a query will be automatically added to the `FROM` clause if not already present. This behavior does not comply with the SQL standard and many people dislike it because it can mask mistakes (such as referencing a table where you should have referenced its alias). The default is `off`. This variable can be enabled for compatibility with releases of PostgreSQL prior to 8.1, where this behavior was allowed by default.

  Note that even when this variable is enabled, a warning message will be emitted for each implicit `FROM` entry referenced by a query. Users are encouraged to update their applications to not rely on this behavior, by adding all tables referenced by a query to the query's `FROM` clause (or its `USING` clause in the case of `DELETE`).

`array_nulls` (`boolean`)

  This controls whether the array input parser recognizes unquoted `NULL` as specifying a null array element. By default, this is `on`, allowing array values containing null values to be entered. However, PostgreSQL versions before 8.2 did not support null values in arrays, and therefore would treat `NULL` as specifying a normal array element with the string value "NULL". For backwards compatibility with applications that require the old behavior, this variable can be turned `off`.

  Note that it is possible to create array values containing null values even when this variable is `off`.

`backslash_quote` (`enum`)

  This controls whether a quote mark can be represented by `\'` in a string literal. The preferred, SQL-standard way to represent a quote mark is by doubling it (`''`) but PostgreSQL has historically also accepted `\'`. However, use of `\'` creates security risks because in some client character set encodings, there are multibyte characters in which the last byte is numerically equivalent to ASCII `\`. If client-side code does escaping incorrectly then a SQL-injection attack is possible. This risk can be prevented by making the server reject queries in which a quote mark appears to be escaped by a backslash. The allowed values of `backslash_quote` are `on` (allow `\'` always), `off` (reject always), and `safe_encoding` (allow only if client encoding does not allow ASCII `\` within a multibyte character). `safe_encoding` is the default setting.

  Note that in a standard-conforming string literal, `\` just means `\` anyway. This parameter affects the handling of non-standard-conforming literals, including escape string syntax (`E'...'`).

`default_with_oids` (`boolean`)

  This controls whether `CREATE TABLE` and `CREATE TABLE AS` include an OID column in newly-created tables, if neither `WITH OIDS` nor `WITHOUT OIDS` is specified. It also determines whether OIDs will be included in tables created by `SELECT INTO`. In PostgreSQL 8.1 `default_with_oids` is `off` by default; in prior versions of PostgreSQL, it was on by default.

  The use of OIDs in user tables is considered deprecated, so most installations should leave this variable disabled. Applications that require OIDs for a particular table should specify `WITH OIDS` when creating the table. This variable can be enabled for compatibility with old applications that do not follow this behavior.

`escape_string_warning` (`boolean`)

  When on, a warning is issued if a backslash (`\`) appears in an ordinary string literal (`'...'` syntax) and `standard_conforming_strings` is off. The default is `on`.

  Applications that wish to use backslash as escape should be modified to use escape string syntax (`E'...'`), because the default behavior of ordinary strings will change in a future release for SQL compatibility. This variable can be enabled to help detect applications that will break.

`regex_flavor` (`enum`)

  The regular expression "flavor" can be set to `advanced`, `extended`, or `basic`. The default is `advanced`. The `extended` setting might be useful for exact backwards compatibility with pre-7.4 releases of PostgreSQL. See Section 9.7.3.1 for details.

`sql_inheritance` (`boolean`)

  This controls the inheritance semantics. If turned `off`, subtables are not included by various commands by default; basically an implied `ONLY` key word. This was added for compatibility with releases prior to 7.1. See Section 5.8 for more information.

`standard_conforming_strings` (`boolean`)

  This controls whether ordinary string literals (`'...'`) treat backslashes literally, as specified in the SQL standard. The default is currently `off`, causing PostgreSQL to have its historical behavior of treating backslashes as escape characters. The default will change to `on` in a future release to improve compatibility with the standard. Applications can check this parameter to determine how string literals will be processed. The presence of this parameter can also be taken as an indication that the escape string syntax (`E'...'`) is supported. Escape string syntax should be used if an application desires backslashes to be treated as escape characters.

`synchronize_seqscans` (`boolean`)

  This allows sequential scans of large tables to synchronize with each other, so that concurrent scans read the same block at about the same time and hence share the I/O workload. When this is enabled, a scan might start in the middle of the table and then "wrap around" the end to cover all rows, so as to synchronize with the activity of scans already in progress. This can result in unpredictable changes in the row ordering returned by queries that have no `ORDER BY` clause. Setting this parameter to `off` ensures the pre-8.3 behavior in which a sequential scan always starts from the beginning of the table. The default is `on`.

## 18.12.2. Platform and Client Compatibility

`transform_null_equals` (`boolean`)

  When on, expressions of the form ``expr` = NULL` (or `NULL = `expr``) are treated as ``expr` IS NULL`, that is, they return true if `expr` evaluates to the null value, and false otherwise. The correct SQL-spec-compliant behavior of ``expr` = NULL` is to always return null (unknown). Therefore this parameter defaults to `off`.

  However, filtered forms in Microsoft Access generate queries that appear to use ``expr` = NULL` to test for null values, so if you use that interface to access the database you might want to turn this option on. Since expressions of the form ``expr` = NULL` always return the null value (using the correct interpretation) they are not very useful and do not appear often in normal applications, so this option does little harm in practice. But new users are frequently confused about the semantics of expressions involving null values, so this option is not on by default.

  Note that this option only affects the exact form `= NULL`, not other comparison operators or other expressions that are computationally equivalent to some expression involving the equals operator (such as `IN`). Thus, this option is not a general fix for bad programming.

  Refer to Section 9.2 for related information.

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