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Tsearch2 - Introduction

[Online version] of this document is available.

The tsearch2 module is available to add as an extension to thePostgreSQL database to allow for Full Text Indexing. This documentis an introduction to installing, configuring, using andmaintaining the database with the tsearch2 module activated.

Please, note, tsearch2 module is fully incompatible with oldtsearch, which is deprecated in 7.4 and will be obsoleted in7.5.

USING TSEARCH2 AND POSTGRESQL FOR A WEB BASED SEARCHENGINE

This documentation is provided as a short guide on how toquickly get up and running with tsearch2 and PostgreSQL, for thosewho want to implement a full text indexed based search engine. Itis not meant to be a complete in-depth guide into the full ins andouts of the contrib/tsearch2 module, and is primarily aimed atbeginners who want to speed up searching of large text fields, orthose migrating from other database systems such as MS-SQL.

The README.tsearch2 file included in the contrib/tsearch2directory contains a brief overview and history behind tsearch.This can also be found online [righthere].

Further in depth documentation such as a full functionreference, and user guide can be found online at the [tsearchdocumentation home].

ACKNOWLEDGEMENTS

Robert John Shepherd originally wrote this documentation for theprevious version of tsearch module (v1) included with the postgresrelease. I took his documentation and updated it to comply with thetsearch2 modifications.

Robert's original acknowledgements:

'Thanks to Oleg Bartunov for taking the time to answer many ofmy questions regarding this module, and also to Teodor Sigaev forclearing up the process of making your own dictionaries. Plus ofcourse a big thanks to the pair of them for writing this module inthe first place!'

I would also like to extend my thanks to the developers, andOleg Bartunov for all of his direction and help with the newfeatures of tsearch2.

OVERVIEW

MS-SQL provides a full text indexing (FTI) system which enablesthe fast searching of text based fields, very useful for websites(and other applications) that require a results set based on keywords. PostgreSQL ships with a contributed module called tsearch2,which implements a special type of index that can also be used forfull text indexing. Further more, unlike MS' offering whichrequires regular incremental rebuilds of the text indexesthemselves, tsearch2 indexes are always up-to-date and keeping themso induces very little overhead.

Before we get into the details, it is recommended that you haveinstalled and tested PostgreSQL, are reasonably familiar withdatabases, the SQL query language and also understand the basics ofconnecting to PostgreSQL from the local shell. This document isn'tintended for the complete PostgreSQL newbie, but anyone with areasonable grasp of the basics should be able to follow it.

INSTALLATION

Starting with PostgreSQL version 7.4 tsearch2 is now included inthe contrib directory with the PostgreSQL sources. contrib/tsearch2is where you will find everything needed to install and usetsearch2. Please note that tsearch2 will also work with PostgreSQLversion 7.3.x, but it is not the module included with the sourcedistribution. You will have to download the module separately andinstall it in the same fashion.

I installed the tsearch2 module to a PostgreSQL 7.3 databasefrom the contrib directory without squashing the original (old)tsearch module. What I did was move the modules tsearch srcdriectory into the contrib tree under the name tsearchV2.

Step one is to download the tsearch V2 module :

[http://www.sai.msu.su/~megera/postgres/gist/tsearch/V2/](check Development History for latest stable version !)

If you are installing from PostgreSQL version 7.4 or higher, youcan skip those steps and just change to the contrib/tsearch2directory in the source tree and continue from there.

As of May 9, 2004 there is a source patch available fortsearch2. The patch provides changes to the pg_ts_ configurationtables to allow for easy dump and restore of a database containingtsearch2. The patch is available here : [http://www.sai.msu.su/~megera/postgres/gist/tsearch/V2/regprocedure_7.4.patch.gz]

To apply this patch, download the mentioned file and place it inyour postgreSQL source tree ($PGSQL_SRC). This patch is notrequired for tsearch2 to work. I would however, highly recommend itas it makes the backup and restore procedures very simple.

If you have a working version of tsearch2 in your database, youdo not need to re-install the tsearch2 module. Just apply the patchand run make. This patch only affects the tsearch2.sql file. Youcan run the SQL script found : [right here] This script will make the modifications found inthe patch, and update the fields from the existing data. From thispoint on, you can dump and restore the database in a normalfashion. Without this patch, you must follow the instructions laterin this document for backup and restore.

This patch is only needed for tsearch2 in PostgreSQL versions7.3.x and 7.4.x. The patch has been applied to the sources for7.5.x.

When you have your source tree for tsearch2 ready, you cancontinue with the regular building and installation process

That is pretty much all you have to do, unless of course you geterrors. However if you get those, you better go check with themailing lists over at http://www.postgresql.org orhttp://openfts.sourceforge.net/since its never failed for me.

If you ever need to revert this patch, and go back to theunpatched version of tsearch2, it is simple if you followed theabove patch command. The -b option creates a backup of the originalfile, so we can just copy it back.

If you need the patched version again, just follow the patchinstructions again.

The directory in the contib/ and the directory from the archiveis called tsearch2. Tsearch2 is completely incompatible with theprevious version of tsearch. This means that both versions can beinstalled into a single database, and migration the new version maybe much easier.

Tsearch2 - Introduction

[Online version] of this document is available.

The tsearch2 module is available to add as an extension to thePostgreSQL database to allow for Full Text Indexing. This documentis an introduction to installing, configuring, using andmaintaining the database with the tsearch2 module activated.

Please, note, tsearch2 module is fully incompatible with oldtsearch, which is deprecated in 7.4 and will be obsoleted in7.5.

USING TSEARCH2 AND POSTGRESQL FOR A WEB BASED SEARCHENGINE

This documentation is provided as a short guide on how toquickly get up and running with tsearch2 and PostgreSQL, for thosewho want to implement a full text indexed based search engine. Itis not meant to be a complete in-depth guide into the full ins andouts of the contrib/tsearch2 module, and is primarily aimed atbeginners who want to speed up searching of large text fields, orthose migrating from other database systems such as MS-SQL.

The README.tsearch2 file included in the contrib/tsearch2directory contains a brief overview and history behind tsearch.This can also be found online [righthere].

Further in depth documentation such as a full functionreference, and user guide can be found online at the [tsearchdocumentation home].

ACKNOWLEDGEMENTS

Robert John Shepherd originally wrote this documentation for theprevious version of tsearch module (v1) included with the postgresrelease. I took his documentation and updated it to comply with thetsearch2 modifications.

Robert's original acknowledgements:

'Thanks to Oleg Bartunov for taking the time to answer many ofmy questions regarding this module, and also to Teodor Sigaev forclearing up the process of making your own dictionaries. Plus ofcourse a big thanks to the pair of them for writing this module inthe first place!'

I would also like to extend my thanks to the developers, andOleg Bartunov for all of his direction and help with the newfeatures of tsearch2.

OVERVIEW

MS-SQL provides a full text indexing (FTI) system which enablesthe fast searching of text based fields, very useful for websites(and other applications) that require a results set based on keywords. PostgreSQL ships with a contributed module called tsearch2,which implements a special type of index that can also be used forfull text indexing. Further more, unlike MS' offering whichrequires regular incremental rebuilds of the text indexesthemselves, tsearch2 indexes are always up-to-date and keeping themso induces very little overhead.

Before we get into the details, it is recommended that you haveinstalled and tested PostgreSQL, are reasonably familiar withdatabases, the SQL query language and also understand the basics ofconnecting to PostgreSQL from the local shell. This document isn'tintended for the complete PostgreSQL newbie, but anyone with areasonable grasp of the basics should be able to follow it.

INSTALLATION

Starting with PostgreSQL version 7.4 tsearch2 is now included inthe contrib directory with the PostgreSQL sources. contrib/tsearch2is where you will find everything needed to install and usetsearch2. Please note that tsearch2 will also work with PostgreSQLversion 7.3.x, but it is not the module included with the sourcedistribution. You will have to download the module separately andinstall it in the same fashion.

I installed the tsearch2 module to a PostgreSQL 7.3 databasefrom the contrib directory without squashing the original (old)tsearch module. What I did was move the modules tsearch srcdriectory into the contrib tree under the name tsearchV2.

Step one is to download the tsearch V2 module :

[http://www.sai.msu.su/~megera/postgres/gist/tsearch/V2/](check Development History for latest stable version !)

If you are installing from PostgreSQL version 7.4 or higher, youcan skip those steps and just change to the contrib/tsearch2directory in the source tree and continue from there.

As of May 9, 2004 there is a source patch available fortsearch2. The patch provides changes to the pg_ts_ configurationtables to allow for easy dump and restore of a database containingtsearch2. The patch is available here : [http://www.sai.msu.su/~megera/postgres/gist/tsearch/V2/regprocedure_7.4.patch.gz]

To apply this patch, download the mentioned file and place it inyour postgreSQL source tree ($PGSQL_SRC). This patch is notrequired for tsearch2 to work. I would however, highly recommend itas it makes the backup and restore procedures very simple.

If you have a working version of tsearch2 in your database, youdo not need to re-install the tsearch2 module. Just apply the patchand run make. This patch only affects the tsearch2.sql file. Youcan run the SQL script found : [right here] This script will make the modifications found inthe patch, and update the fields from the existing data. From thispoint on, you can dump and restore the database in a normalfashion. Without this patch, you must follow the instructions laterin this document for backup and restore.

This patch is only needed for tsearch2 in PostgreSQL versions7.3.x and 7.4.x. The patch has been applied to the sources for7.5.x.

When you have your source tree for tsearch2 ready, you cancontinue with the regular building and installation process

That is pretty much all you have to do, unless of course you geterrors. However if you get those, you better go check with themailing lists over at http://www.postgresql.org orhttp://openfts.sourceforge.net/since its never failed for me.

If you ever need to revert this patch, and go back to theunpatched version of tsearch2, it is simple if you followed theabove patch command. The -b option creates a backup of the originalfile, so we can just copy it back.

If you need the patched version again, just follow the patchinstructions again.

The directory in the contib/ and the directory from the archiveis called tsearch2. Tsearch2 is completely incompatible with theprevious version of tsearch. This means that both versions can beinstalled into a single database, and migration the new version maybe much easier.

NOTE: the previous version of tsearch found in thecontrib/tsearch directory is depricated. Although it is stillavailable and included within PostgreSQL version 7.4. It will beremoved in version 7.5.

ADDING TSEARCH2 FUNCTIONALITY TO A DATABASE

We should create a database to use as an example for theremainder of this file. We can call the database 'ftstest'. You cancreate it from the command line like this:

If you thought installation was easy, this next bit is eveneasier. Change to the PGSQL_SRC/contrib/tsearch2 directory andtype:

The file 'tsearch2.sql' holds all the wonderful little goodiesyou need to do full text indexing. It defines numerous functionsand operators, and creates the needed tables in the database. Therewill be 4 new tables created after running the tsearch2.sql file :pg_ts_dict, pg_ts_parser, pg_ts_cfg, pg_ts_cfgmap are added.

You can check out the tables if you like:

TYPES AND FUNCTIONS PROVIDED BY TSEARCH2

The first thing we can do is try out some of the types that areprovided for us. Lets look at the tsvector type provided forus:

The results are the words used within our string. Notice theyare not in any particular order. The tsvector type returns a stringof space separated words.

Notice the results string has each unique word ('first' and'string' only appear once in the tsvector value). Which of coursemakes sense if you are searching the full text .. you only need toknow each unique word in the text.

Those examples were just casting a text field to that of typetsvector. Lets check out one of the new functions created by thetsearch2 module.

The function to_tsvector has 3 possible signatures:

We will use the second method using two text fields. Theoverloaded methods provide us with a way to specifiy the way thesearchable text is broken up into words (Stemming process). Rightnow we will specify the 'default' configuration. See the section onTSEARCH2 CONFIGURATION to learn more about this.

The result returned from this function is of type tsvector. Theresults came about by this reasoning: All of the words in the textpassed in are stemmed, or not used because they are stop wordsdefined in our configuration. Each lower case morphed word isreturned with all of the positons in the text.

In this case the word 'Our' is a stop word in the defaultconfiguration. That means it will not be included in the result.The word 'first' is found at positions 2 and 6 (although 'Our' is astop word, it's position is maintained). The word(s) positioning ismaintained exactly as in the original string. The word 'used' ismorphed to the word 'use' based on the default configuration forword stemming, and is found at position 4. The rest of the resultsfollow the same logic. Just a reminder again .. the order of the'word' position in the output is not in any kind of order. (ie'use':4 appears first)

If you want to view the output of the tsvector fields withouttheir positions, you can do so with the function'strip(tsvector)'.

If you wish to know the number of unique words returned in thetsvector you can do so by using the function 'length(tsvector)'

Lets take a look at the function to_tsquery. It also has 3signatures which follow the same rational as the to_tsvectorfunction:

Lets try using the function with a single word :

I call the function the same way I would a to_tsvector function,specifying the 'default' configuration for morphing, and the resultis the stemmed output 'word'.

Lets attempt to use the function with a string of multiplewords:

The function can not accept a space separated string. Theintention of the to_tsquery function is to return a type of'tsquery' used for searching a tsvector field. What we need to dois search for one to many words with some kind of logic (for nowsimple boolean).

Notice that the words are separated by the boolean logic 'OR',the text could contain boolean operators &,|,!,() with theirusual meaning.

You can not use words defined as being a stop word in yourconfiguration. The function will not fail .. you will just get noresult, and a NOTICE like this:

That is a beginning to using the types, and functions defined inthe tsearch2 module. There are numerous more functions that I havenot touched on. You can read through the tsearch2.sql file builtwhen compiling to get more familiar with what is included.

INDEXING FIELDS IN A TABLE

The next stage is to add a full text index to an existing table.In this example we already have a table defined as follows:

We are assuming there are several rows with some kind of data inthem. Any data will do, just do several inserts with test stringsfor a topic, and a message. here is some test data I inserted. (yesI know it's completely useless stuff ;-) but it will serve ourpurpose right now).

The next stage is to create a special text index which we willuse for FTI, so we can search our table of messages for words or aphrase. We do this using the SQL command:

Note that unlike traditional indexes, this is actually a newfield in the same table, which is then used (through the magic ofthe tsearch2 operators and functions) by a special index we willcreate in a moment.

The general rule for the initial insertion of data will followfour steps:

The data can be updated into the table, the vacuum full analyzewill reclaim unused space. The index can be created on the tableafter the data has been inserted. Having the index created prior tothe update will slow down the process. It can be done in thatmanner, this way is just more efficient. After the index has beencreated on the table, vacuum full analyze is run again to updatepostgres's statistics (ie having the index take effect).

Note that this only inserts the field strMessage as a tsvector,so if you want to also add strTopic to the information stored, youshould instead do the following, which effectively concatenates thetwo fields into one before being inserted into the table:

Using the coalesce function makes sure thisconcatenation also works with NULL fields.

We need to create the index on the column idxFTI. Keep in mindthat the database will update the index when some action is taken.In this case we _need_ the index (The whole point of Full TextINDEXINGi ;-)), so don't worry about any indexing overhead. We willcreate an index based on the gist function. GiST is an indexstructure for Generalized Search Tree.

After you have converted all of your data and indexed thecolumn, you can select some rows to see what actually happened. Iwill not display output here but you can play around yourselves andsee what happened.

The last thing to do is set up a trigger so every time a row inthis table is changed, the text index is automatically updated.This is easily done using:

Or if you are indexing both strMessage and strTopic you shouldinstead do:

Before you ask, the tsearch2 function accepts multiple fields asarguments so there is no need to concatenate the two into one likewe did before.

If you want to do something specific with columns, you may writeyour very own trigger function using plpgsql or other procedurallanguages (but not SQL, unfortunately) and use it instead oftsearch2 trigger.

You could however call other stored procedures from within thetsearch2 function. Lets say we want to create a function to removecertain characters (like the @ symbol from all text).

Now we can use this function within the tsearch2 function on thetrigger.

If at this point you receive an error stating: ERROR: Can't findtsearch config by locale

Do not worry. You have done nothing wrong. And tsearch2 is notbroken. All that has happened here is that the configuration issetup to use a configuration based on the locale of the server. Allyou have to do is change your default configuration, or add a newone for your specific locale. See the section on TSEARCH2CONFIGURATION.

This could be useful for removing other characters from indexedtext, or any kind of preprocessing needed to be done on the textprior to insertion into the index.

QUERYING A TABLE

There are some examples in the README.tsearch2 file for queryinga table. One major difference between tsearch and tsearch2 is theoperator ## is no longer available. Only the operator @@ isdefined, using the types tsvector on one side and tsquery on theother side.

Lets search the indexed data for the word 'Test'. I indexedbased on the the concatenation of the strTopic, and thestrMessage:

The only result that matched was the row with a topic 'TestingTopic'. Notice that the word I search for was all lowercase. Let'ssee what happens when I query for uppercase 'Test'.

We get zero rows returned. The reason is because when the textwas inserted, it was morphed to my default configuration (becauseof the call to to_tsvector in the UPDATE statement). If there wasno morphing done, and the tsvector field(s) contained the word'Text', a match would have been found.

Download free windows 7 64 bit iso. Most likely the best way to query the field is to use theto_tsquery function on the right hand side of the @@ operator likethis:

That query searched for all instances of 'Test' OR 'Zeppelin'.It returned two rows: the 'Testing Topic' row, and the 'ClassicRock Bands' row. The to_tsquery function performed the correctmorphology upon the parameters, and searched the tsvector fieldappropriately.

The last example here relates to searching for a phrase, forexample 'minority report'. This poses a problem with regard totsearch2, as it doesn't index phrases, only words. But there is away around which doesn't appear to have a significant impact onquery time, and that is to use a query such as the following:

Of course if your indexing both strTopic and strMessage, andwant to search for this phrase on both, then you will have to getout the brackets and extend this query a little more.

TSEARCH2 CONFIGURATION

Some words such as 'and', 'the', and 'who' are automatically notindexed, since they belong to a pre-existing dictionary of 'StopWords' which tsearch2 does not perform indexing on. If someoneneeds to search for 'The Who' in your database, they are going tohave a tough time coming up with any results, since both areignored in the indexes. But there is a solution.

Lets say we want to add a word into the stop word list forenglish stemming. We could edit the file:'/usr/local/pgsql/share/english.stop' and add a word to the list.I edited mine to exclude my name from indexing:

When you connect to the database, the dict_init procedure is runduring initialization. And in my configuration it will read thestop words from the file I just edited. If you were connected tothe DB while editing the stop words, you will need to end thecurrent session and re-connect. When you re-connect to thedatabase, 'andy' is no longer indexed:

Originally I would get the result :

But since I added it as a stop word, it would be ingnored on theindexing. The stop word added was used in the dictionary 'en_stem'.If I were to use a different configuration such as 'simple', theresults would be different. There are no stop words for the simpledictionary. It will just convert to lower case, and index everyunique word.

All this talk about which configuration to use is leading usinto the actual configuration of tsearch2. In the examples in thisdocument the configuration has always been specified when using thetsearch2 functions:

The pg_ts_cfg table holds each configuration you can use withthe tsearch2 functions. As you can see the ts_name column containsboth the 'default' configurations based on the 'C' locale. And the'simple' configuration which is not based on any locale.

Each row in the pg_ts_cfg table contains the name of thetsearch2 configuration, the name of the parser to use, and thelocale mapped to the configuration. There is only one parser tochoose from the table pg_ts_parser called 'default'. More parserscould be written, but for our needs we will use the default.

There are 3 configurations installed by tsearch2 initially. Ifyour locale is set to 'en_US' for example (like my laptop), then asyou can see there is currently no dictionary configured to use withthat locale. You can either set up a new configuration or just useone that already exists. If I do not specify which configuration touse in the to_tsvector function, I receive the following error.

We will create a new configuration for use with the serverencoding 'en_US'. The first step is to add a new configuration intothe pg_ts_cfg table. We will call the configuration'default_english', with the default parser and use the locale'en_US'.

We have only declared that there is a configuration called'default_english'. We need to set the configuration of how'default_english' will work. The next step is creating a newdictionary to use. The configuration of the dictionary iscompletlely different in tsearch2. In the prior versions to makechanges, you would have to re-compile your changes into thetsearch.so. All of the configuration has now been moved into thesystem tables created by executing the SQL code fromtsearch2.sql

Lets take a first look at the pg_ts_dict table

The dict_name column is the name of the dictionary, for example'simple', 'en_stem' or 'ru_stem'. The dict_init column is a textrepresentation of a stored procedure to run for initialization ofthat dictionary, for example 'snb_en_init(text)' or'snb_ru_init(text)'. The initial configuration of tsearch2 had thedict_init and dict_lexize columns as type oid. The patch mentionedin the Installation Notes changes these types to regprocedure. Thedata inserted, or updated can still be the oid of the storedprocedure. The representation is just different. This makes backupand restore procedures much easier for tsearch2. The dict_initoption is used for options passed to the init function for thestored procedure. In the cases of 'en_stem' or 'ru_stem' it is apath to a stopword file for that dictionary, for example'/usr/local/pgsql/share/english.stop'. This is however dictated bythe dictionary. ISpell dictionaries may require different options.The dict_lexize column is another OID of a stored procedure to thefunction used to lexize, for example 'snb_lexize(internal,internal, integer)'. The dict_comment column is just a comment.

Next we will configure the use of a new dictionary based onISpell. We will assume you have ISpell installed on you machine.(in /usr/local/lib)

There has been some confusion in the past as to which files areused from ISpell. ISpell operates using a hash file. This is abinary file created by the ISpell command line utility 'buildhash'.This utility accepts a file containing the words from thedictionary, and the affixes file and the output is the hash file.The default installation of ISPell installs the english hash fileenglish.hash, which is the exact same file as american.hash. ISpelluses this as the fallback dictionary to use.

This hash file is not what tsearch2 requires as the ISpellinterface. The file(s) needed are those used to create the hash.Tsearch uses the dictionary words for morphology, so the listing isneeded not spellchecking. Regardless, these files are included inthe ISpell sources, and you can use them to integrate intotsearch2. This is not complicated, but is not very obvious to beginwith. The tsearch2 ISpell interface needs only the listing ofdictionary words, it will parse and load those words, and use theISpell dictionary for lexem processing.

I found the ISPell make system to be very finicky. Theirdocumentation actually states this to be the case. So I just didthings the command line way. In the ISpell source tree underlangauges/english there are several files in this directory. For acomplete description, please read the ISpell README. Basically forthe english dictionary there is the option to create the small,medium, large and extra large dictionaries. The medium dictionaryis recommended. If the make system is configured correctly, itwould build and install the english.has file from the medium sizedictionary. Since we are only concerned with the dictionary wordlisting .. it can be created from the /languages/english directorywith the following command:

This will create a file called english.med. You can copy thisfile to whever you like. I placed mine in /usr/local/lib so itcoincides with the ISpell hash files. You can now add the tsearch2configuration entry for the ISpell english dictionary. We will alsocontinue to use the english word stop file that was installed forthe en_stem dictionary. You could use a different one if you like.The ISpell configuration is based on the 'ispell_template'dictionary installed by default with tsearch2. We will use the OIDsto the stored procedures from the row where the dict_name ='ispell_template'.

Now that we have a dictionary we can specify it's use in a queryto get a lexem. For this we will use the lexize function. Thelexize function takes the name of the dictionary to use as anargument. Just as the other tsearch2 functions operate. You willneed to stop your psql session and start it again in order for thismodification to take place.

If you wanted to always use the ISpell english dictionary youhave installed, you can configure tsearch2 to always use a specificdictionary.

Lexize is meant to turn a word into a lexem. It is possible toreceive more than one lexem returned for a single word.

The lexize function is not meant to take a full string as anargument to return lexems for. If you passed in an entire sentence,it attempts to find that entire sentence in the dictionary. Sincethe dictionary contains only words, you will receive an emptyresult set back.

This is not to say that tsearch will be ignored when adding textinformation to the the tsvector index column. This will beexplained in greater detail with the table pg_ts_cfgmap.

Next we need to set up the configuration for mapping thedictionay use to the lexxem parsings. This will be done by alteringthe pg_ts_cfgmap table. We will insert several rows, specifying touse the new dictionary we installed and configured for lexizingwithin tsearch2. There are several type of lexims we would beconcerned with forcing the use of the ISpell dictionary.

We have just inserted 3 records to the configuration mapping,specifying that the lexem types for 'lhword, lpart_hword and lword'are to be stemmed using the 'en_ispell' dictionary we added intopg_ts_dict, when using the configuration ' default_english' whichwe added to pg_ts_cfg.

There are several other lexem types used that we do not need tospecify as using the ISpell dictionary. We can simply insert valuesusing the 'simple' stemming process dictionary.

Our addition of a configuration for 'default_english' is nowcomplete. We have successfully created a new tsearch2configuration. At the same time we have also set the newconfiguration to be our default for en_US locale.

Notice here that words like 'tsearch' are still parsed andindexed in the tsvector column. There is a lexem returned for theword becuase in the configuration mapping table, we specify wordsto be used from the 'en_ispell' dictionary first, but as a fallbackto use the 'en_stem' dictionary. Therefore a lexem is not returnedfrom en_ispell, but is returned from en_stem, and added to thetsvector.

Notice in this last example I added the word 'computer' to thetext to be converted into a tsvector. Because we have setup ourdefault configuration to use the ISpell english dictionary, thewords are lexized, and computer returns 2 lexems at the sameposition. 'compute':7 and 'computer':7 are now both indexed for theword computer.

You can create additional dictionary lists, or use the extralarge dictionary from ISpell. You can read through the ISpelldocuments, and source tree to make modifications as you seefit.

In the case that you already have a configuration set for thelocale, and you are changing it to your new dictionaryconfiguration. You will have to set the old locale to NULL. If weare using the 'C' locale then we would do this:

That about wraps up the configuration of tsearch2. There is muchmore you can do with the tables provided. This was just anintroduction to get things working rather quickly.

ADDING NEW DICTIONARIES TO TSEARCH2

To aid in the addition of new dictionaries to the tsearch2module you can use another additional module in combination withtsearch2. The gendict module is included into tsearch2 distributionand is available from gendict/ subdirectory.

I will not go into detail about installation and instructions onhow to use gendict to it's fullest extent right now. You can readthe README.gendict .. it has all of the instructions andinformation you will need.

BACKING UP AND RESTORING DATABASES THAT FEATURE TSEARCH2

Never rely on anyone elses instructions to backup andrestore a database system, always develop and understand your ownmethodology, and test it numerous times before you need to do itfor real.

The backup and restore procedure has changed over time. This isnot meant to be the bible for tsearch2 back up and restore. Pleaseread all sections so you have a complete understanding of somebackup and restore issues. Please test your own procedures, and donot rely on these instructions solely.

If you come accross some issues in your own procedures, pleasefeel free to bring the question up on the Open-FTS, and PostgreSQLmailing lists.

ORIGINAL BACKUP PROCEDURES

Originally, tsearch2 had problems when using the pg_dump, and orthe pg_dumpall utilities. The problem lies within the original useof OIDs for column types. Since OIDs are not consistent accrosspg_dumps, when you reload the data values into the pg_ts_dicttable, for example, those oids no longer point to anything. Youwould then end up trying to use a 'broken' tsearch2configuration.

The solution was to backup and restore a database using thetsearch2 module into small unique parts, and then load them in thecorrect order. You would have to edit the schema and remove thetsearch stored procedure references in the sql file. You would haveto load your global objects, then the tsearch2 objects. You had tore-create the tsearch module before restoring your schema so noconflicts would arise. Then you could restore your data (allschemas, and types needed for the data were now available).

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The original backup instructions were asfollows

1) Backup any global database objects such as users and groups(this step is usually only necessary when you will be restoring toa virgin system)

2) Backup the full database schema using pg_dump

3) Backup the full database using pg_dump

The original restore procedures were asfollows

1) Create the blank database

2) Restore any global database objects such as users and groups(this step is usually only necessary when you will be restoring toa virgin system)

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3) Create the tsearch2 objects, functions and operators

4) Edit the backed up database schema and delete all SQLcommands which create tsearch2 related functions, operators anddata types, BUT NOT fields in table definitions that specifytsvector types. If your not sure what these are, they are the oneslisted in tsearch2.sql. Then restore the edited schema to thedatabase

5) Restore the data for the database

If you get any errors in step 4, it will most likely be becauseyou forgot to remove an object that was created in tsearch2.sql.Any errors in step 5 will mean the database schema was probablyrestored wrongly.

Issues with this procedure

As I mentioned before, it is vital that you test out your ownbackup and restore procedures. These procedures were originallyadopted from this document's orignal author. Robert John Shepherd.It makes use of the pg_dump custom archive functionality. I am notthat familiar with the formatting output of pg_dump, and usingpg_restore. I have always had the luxury of using text files(Everything is DATABASE.sql).

One issue not forseen in the case of using a binary dump is thewhen you have added more than the default tsearch2 configurations.Upon reload of the data it will fail due to duplicate primary keys.If you load the tsearch2 module, and then delete the data loaded bytsearch2 into the configuration tables, the data will restore. Theconfigurations are incorrect because you can not remove the datausing OID references from the custom archive.

It would be very simple to fix this problem if the data was notin an archive format. I do believe all of your data would have beenrestored properly and you can get things working fairly easy. Allone would have to do is create the configurations as in thetsearch2.sql file. And then create your custom configurationsagain.

I have read in the pg_dump man page that if the tar archiveformat is used, it is possible to limit which data is restoredusing pg_restore. If anyone has more experience with pg_dumparchives, and pg_restore. Please feel free to test and contributeyour procedure(s).

CURRENT BACKUP AND RESTORE PROCEDURES

Currently a backup and restore of a database using the tsearch2module can be quite simple. If you have applied the patch mentionedin the installation instructions prior to tsearch2 installation.This patch removes the use of the oid column. The textrepresentation for the stored procedures used are dumped with thedata and the restoration of the data works seemlessly.

1) to backup the database

1) to restore the database

This procedure is now like any normal backup and restoreprocedure. I can say whether this has been proven using the pg_dumparchive, and restoring with pg_restore. In theory there should beno problems with any format after the patch is applied.

This restoration procedure should never be an issue with thepatch applied to version 7.5 of PostgreSQL. Only versions 7.3 and7.4 are affected. You can avoid any troubles by applying the patchprior to installation, or running the SQL script provided to livedatabase before backup and restoring is done.