Traditionally, Advantage has been able to use one collation at a time for ordering character data. With Advantage Database Server, a collation for ANSI character data and a collation for OEM character data are stamped into the server binary. These collations came from ansi.chr and extend.chr (for OEM character data). With Advantage Local Server, these collations are specified in the local server configuration file and are loaded at application startup. These are static collations because they cannot be changed at runtime. The character type specified with the table or SQL statement (ADS_ANSI or ADS_OEM) specifies if the ANSI collation is used or if the OEM collation is used. Note that ADT tables only use the ANSI collation.
Beginning with version 9.0, additional collation support has been added to provide compatibility with Visual FoxPro tables. These collations are loaded dynamically at runtime as they are needed, which provides for the ability for an application to switch between different collations as desired and for a single table to have indexes built with different collations. The dynamic collations can be used both with Visual FoxPro (VFP) tables and with Advantage Proprietary (ADT) tables.
Using a different collation allows for data to be sorted according to the rules of a certain locale, and it allows for string comparisons (e.g., in filters, and SQL WHERE clauses) to match according to different rules. The static collations that are stamped into the server or specified in the local server configuration file provide a one-to-one mapping of characters to unique collation values. Each of the 256 character values maps to a different collation value. The dynamic collations provide for more flexibility in collation. For example, the German collation for code page 1252 (GERMAN_VFP_CI_AS_1252) accounts for expansion characters such as ‘æ’ and ‘ß’, which will collate the same as the corresponding two-character sequences ‘ae’ and ‘ss’. This means, for example, that the words ‘Strasse’ and ‘Straße’ are considered equivalent for comparison and sorting purposes (a filter with one of those values will find records containing either value). Also, contraction character sequences in collations such as SPANISH_VFP_CI_AS_850 are handled correctly. For example, the character pair ‘ch’ is treated as a single collation element for sorting purposes (it collates between ‘c’ and ‘d’).
In addition, accented characters are treated differently in some of the dynamic collations. The accented characters have unique collation values from the non-accented characters, but they are differentiated with secondary collation weights, which provides for a more logical sorting of data. For example, with a single-byte static collation, the following words would be sorted:
resume, resumes, resumé, resumés
A dynamic collation such as GENERAL_VFP_CI_AS_1252 will order the words as:
resume, resumé, resumes, resumés
Many of the new Visual FoxPro-compatible collations require two bytes of collation data per character. This means that an index built with one of these collations will take more space, although the extra space may not be significant because of the index compression techniques that are used.
The collation names provide information about the collation itself. They are based on the names used by Visual FoxPro with an appended _VFP_ in the name to indicate that it is compatible with Visual FoxPro. Collations with _CI_ in the name are case-insensitive; _CS_ indicates it is case-sensitive. If the name contains _AS_, the collation is accent sensitive, and _AI_ indicates it is accent-insensitive, although the copy of adscollate.adt available with version 9.0 does not yet have any accent-insensitive collations. The final part of the name is the codepage for which the collation is intended. The "machine" collations (e.g., MACHINE_VFP_BIN_1252) are single-byte collations that all order data in byte order based on the ASCII value of the character. The _BIN_ in the name signifies raw binary sorting. The machine collations are both case-sensitive and accent-sensitive.
The currently available Visual FoxPro-compatible collations include CZECH, DUTCH, GENERAL, GERMAN, GREEK, HUNGARY, ICELAND, MACHINE, NORDAN, POLISH, RUSSIAN, SLOVAK, SPANISH, SWEFIN, TURKISH, and UNIQWT. They include support for code pages 437, 620, 737, 850, 852, 857, 861, 865, 866, 895, 1250, 1251, 1252, 1253, and 1254. At this point none of the collations for multi-byte character sets are supported. Specifically, collations built in Visual FoxPro that use the National Language Support (NLS) will not be compatible with Advantage.
The collations are physically stored in the files adscollate.adt and adscollate.adm. The file adscollate.adt is a standard Advantage proprietary format (ADT) table that can be opened the same as any other ADT table. However, you can use the system procedure sp_GetCollations to retrieve the list of available collations along with short text descriptions and other information. In order to use these dynamic collations, adscollate.adt and the associated memo file adscollate.adm must be available for Advantage Database Server or Advantage Local Server to load at runtime. When using Advantage Database Server the file is expected to reside in the same directory as the ADS binary (e.g., ads.exe, ads.nlm, libadsd.so). With Advantage Local Server, the file must be available in the search path; for example, it would normally be in the same directory as the adslocal.cfg file.
In order to use one of the dynamically loaded collations, you must specify the collation name at run time. In general, the collation name is specified instead of the existing ADS_ANSI or ADS_OEM character set choice. The method depends on the development environment you are using. When using one of the connection-string oriented clients such as JDBC, OLE DB, ADO.NET, etc. the application should specify the collation name in the connection string. For example, specifying ‘chartype=general_vfp_ci_as_1252’ in the connection string when using the Advantage .NET Data Provider will cause all operations in that connection to use the GENERAL collation for codepage 1252. A Delphi application, for example, can specify one of these collations via the AdsCharType property.
In general, choosing ANSI (ADS_ANSI) or OEM (ADS_OEM) chooses one of the static single-byte collations that is stamped in the Advantage Database Server or specified in adslocal.cfg. Choosing another name specifies that the named collation is to be loaded dynamically at runtime from adscollate.adt.
There are some runtime differences between the traditional static collations and the dynamic collations. Aside from being able to choose different collations at runtime, the primary difference is that indexes built with the dynamic collations will be maintained no matter which collation is currently active in the application. If an index is built with a static collation, then that collation must always be the one available in the server, otherwise the index must be rebuilt to avoid logical index corruption.
A benefit to using a static collation is that you can install a version of Advantage Database Server with the desired collation language and rebuild all indexes (or use the auto-creation option), and your application will automatically use that new collation. In other words, some of the localization can be handled at install time. In contrast, an index built with one of the dynamic collations must be dropped and created again in order to use a different collation. With dynamic collations, the name of the collation is stored in the index header in the index file itself.
Advantage will only use a dynamic collation for optimization purposes if the collation matches the currently chosen collation. For example, if the collation currently in use for an SQL statement is HUNGARY_VFP_CI_AS_1250, then it will only use an index built on a character field for optimizing an ORDER BY clause or a filter if the index was built with that same collation. Note that in a development environment that allows you to choose indexes directly for ordering results (e.g., with Delphi and the TAdsTable component), you can choose any index to be the active index even if it does not match the current collation.
The handling of Full Text Search (FTS) indexes is a little different. When using the CONTAINS() function to do an FTS search, Advantage will use any FTS index available even if it was built with a different collation. Another difference with FTS indexes is that accent information is not stored when a dynamic collation with secondary weight information is used. For example, if the collation GENERAL_VFP_CI_AS_1252 is used for building an FTS index, the accent information for characters such as á, è, ö, etc. is not stored in the index. This means that a search such as CONTAINS(*, ‘resume’) will find resumé. If this is not desired, then it is necessary to use a static collation or use one of the MACHINE dynamic collations.
Because of the fact that multiple dynamic collations can be specified and in use at any one time, it presents the possibility of silently hiding optimization problems. For example, if you write an SQL query in Advantage Data Architect and look at the optimization plan, you might see that it is desirable to add a new index. If your application specifies a dynamic collation that is not currently chosen in Advantage Data Architect, then the new index will be built with a collation that is not used by the application. If you then run the query in your application, it might not be optimized because the collation would not match that of the new index.
To help make developers aware of this potential issue, Advantage will, by default, produce a 5209 error when a table is opened that has indexes built with collations that do not match the currently specified collation.
If you want to be able to use multiple collations concurrently on a single table, you can use the system procedure sp_AllowMultipleCollations to turn off the 5209 error. The information that controls whether this error is generated is contained in the adscollate.adt collation table repository. This means that if you turn off the 5209 error for one or more collations, it will turn it off for all applications that are using that same collation repository.