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129 lines
4.5 KiB
Markdown
129 lines
4.5 KiB
Markdown
# SQL Language Extension: SET BIND
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## Implements capability to setup columns coercion rules in current session.
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### Author:
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Alex Peshkoff <peshkoff@mail.ru>
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### Syntax is:
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```sql
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SET BIND OF { type-from | TIME ZONE } TO { type-to | LEGACY | EXTENDED | NATIVE };
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```
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### Description:
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Makes it possible to define rules of describing types of returned to the client columns in non-standard way -
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`type-from` is replaced with `type-to`, automatic data coercion takes place.
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When incomplete type definition is used (i.e. `CHAR` instead `CHAR(n)`) in left part of `SET BIND` coercion
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will take place for all `CHAR` columns, not only default `CHAR(1)`. Special incomplete type `TIME ZONE`
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stands for all types (namely TIME & TIMESTAMP) WITH TIME ZONE.
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When incomplete type definiton is used in right side of the statement (TO part) firebird engine will define missing
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details about that type automatically based on source column.
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Changing bind of any NUMERIC/DECIMAL does not affect appropriate underlying integer type. On contrary,
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changing bind of integer datatype also affects appropriate NUMERICs/DECIMALs.
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Special `TO` part format `LEGACY` is used when datatype, missing in previous FB version, should be represented in
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a way, understandable by old client software (may be with some data losses). The following coercions are done for
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legacy datatypes:
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| Native datatype | Legacy datatype |
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|--------------------------|-----------------------------|
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| BOOLEAN | CHAR(5) |
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| DECFLOAT | DOUBLE PRECISION |
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| INT128 | BIGINT |
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| TIME WITH TIME ZONE | TIME WITHOUT TIME ZONE |
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| TIMESTAMP WITH TIME ZONE | TIMESTAMP WITHOUT TIME ZONE |
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Using `EXTENDED` in the `TO` part directs firebird engine to coerce to extended form of `FROM` datatype.
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Currently it works only for TIME/TIMESTAMP WITH TIME ZONE - they are coerced to EXTENDED TIME/TIMESTAMP WITH TIME ZONE
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appropriately.
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Setting `NATIVE` means `type` will be used as if there were no previous rules for it.
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Except SQL-statement there are two more ways to specify data coercion - tag `isc_dpb_set_bind` in DPB
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and `DataTypeCompatibility` parameter in firebird.conf & databases.conf. The later the rule is introduced
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(.conf->DPB->SQL) the higher priotiy it has.
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I.e. one can override .conf in any other way and DPB from SQL statement.
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Value of clumplet with `isc_dpb_set_bind` tag in DPB should be specified as a set of partially
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formed SET BIND statements, i.e. with prefix SET BIND OF is omitted, separated by semicolons.
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For example:
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```c++
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dpb->insertString(&status, isc_dpb_set_bind, "decfloat to char; int128 to char");
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```
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`DataTypeCompatibility` is minor firebird version for which we want to provide some compatibility
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regarding data types. That compatibility may be not absolute - for example SET BIND can't care about type
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of particular SQL functions. The following types will be described in legacy form when `DataTypeCompatibility=3.0`:
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DECFLOAT, INT128 and TIME(STAMP) WITH TIME ZONE. When `DataTypeCompatibility=2.5` in addition to this list
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BOOLEAN will be described as legacy type as well.
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Only fields returned by database engine in regular messages are modified according to SET BIND rules.
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Variables returned by getting slice of an array are not affected by SET BIND statement.
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### SQL Samples:
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```sql
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SELECT CAST('123.45' AS DECFLOAT(16)) FROM RDB$DATABASE; --native
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```
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```
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CAST
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=======================
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123.45
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```
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```sql
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SET BIND OF DECFLOAT TO DOUBLE PRECISION;
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SELECT CAST('123.45' AS DECFLOAT(16)) FROM RDB$DATABASE; --double
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```
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```
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CAST
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=======================
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123.4500000000000
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```
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```sql
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SET BIND OF DECFLOAT(34) TO CHAR;
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SELECT CAST('123.45' AS DECFLOAT(16)) FROM RDB$DATABASE; --still double
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```
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```
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CAST
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=======================
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123.4500000000000
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```
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```sql
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SELECT CAST('123.45' AS DECFLOAT(34)) FROM RDB$DATABASE; --text
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```
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```
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CAST
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==========================================
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123.45
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```
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In a case of missing ICU on client:
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```sql
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SELECT CURRENT_TIMESTAMP FROM RDB$DATABASE;
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```
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```
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CURRENT_TIMESTAMP
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=========================================================
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2020-02-21 16:26:48.0230 GMT*
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```
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```sql
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SET BIND OF TIME ZONE TO EXTENDED;
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SELECT CURRENT_TIMESTAMP FROM RDB$DATABASE;
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```
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```
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CURRENT_TIMESTAMP
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=========================================================
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2020-02-21 19:26:55.6820 +03:00
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```
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