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PostgreSQL: A full text search engine - Part 2

Welcome to the second installment of our look into full text search within PostgreSQL.

If this is the first time you heard about full text search I highly encourage you to go and read the first chapter in this series before continuing. This chapter builds on what we have seen previously.

A look back

In short, the previous chapter introduced the general concept of full text search, regardless of the software being used. It looked at how the idea of full text search was brought to computer software by breaking it up into roughly three steps: case removal, stop word removal, normalizing with synonyms and stemming.

Next we delved into PostgreSQL's implementation and introduced the tsvector and the tsquery as two new data types together with a handful of new functions such as to_tsvector(), to_tsquery() and plainto_tsquery(), which all extend PostgreSQL to support full text search.

We saw how we could feed PostgreSQL a string of text which would then get parsed into tokens and processed even further into lexemes which in turn got stored into a tsvector. We then queried that tsvector using the tsquery data type and the @@ matching operator.

In this chapter, I want to flesh out an important topic we touched on in previously: PostgreSQL's full text search configurations.

Precaution

Let me be very clear, in most cases the configurations shipped with PostgreSQL will suffice and you do not need to touch them at all, in which case this chapter could be considered a waste of time.

However, I highly encourage you to read through this chapter and, as always, actually run the queries with me. You need to know how the tools you use work under the hood.

To be even more bold, someday you might even need to get your hands dirty and actually build your own configuration. Why? Because a customer wanting full text search for their application might have specific requirements, or even deliver you specific dictionaries to use in the parsing stage. Such use cases may arise in very specific areas of conduct where much official, technical lingo is used which is not covered in a general dictionary.

So, put on your favorite pants (or none if you like that better), turn down the lights, pull the computer close to you, open up a terminal window, put on some eery music and let us get started.

Configuring PostgreSQL full text search

In the last chapter we saw that PostgreSQL uses a couple of tools like a stop word list and dictionaries to perform its parsing. We also saw that we did not need to tell PostgreSQL about which of these tools to use. It turned out that full text search comes with a set of default configurations for several languages. We also found out that, if no configuration was given, the database assumes that the document or string to be parsed is English and uses a configuration called 'english'.

Beware of localized packages of PostgreSQL though. As I noted in the previous chapter, there is a small possibility that the default configuration in your PostgreSQL installation is not set to 'english'. If this is the case with your setup, be sure to include the 'english' configuration if not stated otherwise or change it to be 'english'. We will see how to do that in a minute.

Taking the small database we created last time, the syntax to feed a configuration set to PostgreSQL during parsing was the following:

SELECT to_tsquery('english','elephants & blue');

The string 'english' represents the name of the configuration which we would like to use. As you know by now, this string can be omitted which will make the database use the default configuration. PostgreSQL knows this default because it is set in the general postgresql.conf configuration file. In that file you will find a variable called default_text_search_config which, in most cases, is set to pg_catalog.english. If you wish to have a own, custom configuration to be the default, that is the place to set it.

Before hacking away at your own configuration, it may be of interest to see what PostgreSQL has to offer. To see which shipped configuration files are available to you, use the describe command (\d) together with the full text flag (F):

\dF

This will describe the objects in the database that represent full text configurations. You see that by default you have quite a lot of language support. To see a different configuration in action, let us do a quick, fun test.

First, take the dutch string "Een blauwe olifant springt al dartelend over de kreupele dolfijn.", which is a rough translation of the "The blue elephant jumps over the crippled dolphin." example from the first chapter. If we would feed this to PostgreSQL, using the default (english) configuration:

SELECT to_tsvector('english', 'Een blauwe olifant springt al dartelend over de kreupele dolfijn');

We would get back:

 'al':5 'blauw':2 'dartelend':6 'de':8 'dolfijn':10 'een':1 'kreupel':9 'olif':3 'springt':4

It attempted to guess some words as you can see from the lexeme 'olif', but, to a dutch reader, this is not stemmed correctly. Neither are the stop words removed: 'de' and 'een' are articles which, in dutch, are considered of no value in a text search context. So let us try this again with the built-in dutch configuration:

SELECT to_tsvector('dutch', 'Een blauwe olifant springt al dartelend over de kreupele dolfijn');

And we get:

 'blauw':2 'dartel':6 'dolfijn':10 'kreupel':9 'olifant':3 'springt':4

Aha! That is much shorter then the previous result, and it is also more correct. As you can see, the words 'de' and 'een' are now removed and the stemming is done correctly on 'dartel', 'olifant' and 'kreupel'. The target of this series, however, is not to show you the dutch language (for it will make you weep...), but you see the effect a different configuration set can have.

But what is such a configuration set made of? To answer that, we can simply use the same describe, but ask for more detailed information with the + flag:

\dF+

This will return a list of all the configurations and their details, so let us filter that and look at only the english version:

\dF+ english

The following result will be returned:

 asciihword      | english_stem
 asciiword       | english_stem
 email           | simple
 file            | simple
 float           | simple
 host            | simple
 hword           | english_stem
 hword_asciipart | english_stem
 hword_numpart   | simple
 hword_part      | english_stem
 int             | simple
 numhword        | simple
 numword         | simple
 sfloat          | simple
 uint            | simple
 url             | simple
 url_path        | simple
 version         | simple
 word            | english_stem

All of these are token categories that target the different groups of words that the PostgreSQL full text parser recognizes. For each category there are one or more dictionaries defined which will receive the token and try to return a lexeme. We also call this overview a configuration map, for it maps a category to one or more dictionaries.

If the parser encounters a URL, for example, it will categorize it as a url or url_path token and as a result, PostgreSQL will consult the dictionaries mapped to this category to try and create a single lexeme containing a URL pointing to the same path. Example:

  • example.com
  • example.com/index.html
  • example.com/foo/../index.html

The URLs all result in the same document being served, so it makes sense to only save one variant as a lexeme in the resulting vector. The same kind of normalization is done for file paths, version numbers, host names, units of measure, ... . A lot more then normal, English words.

There are 23 categories in total that the parser can recognize, ones not included here, for example, are tag for XML tags, blank for whitespace or punctuation, etc.

To see a description of the different token categories supported, use the 'p' flag together with '+' for more information:

\dFp+

When parsing, the chain of command goes as follows:

  • A string is fed to PostgreSQL's full text
  • The parser crawls over the string and chops it into tokens of a certain type
  • For each token category a list of dictionaries (or a single dictionary) is consulted
  • If a dictionary list is used, the dictionaries are (generally) ordered from most precise (narrow) to most generic (wide)
  • As soon as a dictionary returns a lexeme (single or in the form of an array), the flow for that token stops
  • If no lexeme is proposed (a dictionary returns NULL) the token is given to the next dictionary in line or if a stop word list returns a match (returns empty array), the token is discarded

Dictionary templates and dictionaries

In the list of token categories that were supported by the built-in 'english' configuration, you will find that only two dictionaries are used: simple and english_stem, which in turn come from the simple and snowball dictionary templates respectively.

So, what exactly is the difference between a dictionary template and a dictionary?

A dictionary template is the skeleton (hence template) of a dictionary. It defines the actual C functions that will do the heavy lifting. A dictionary is an instantiation of that template - providing it with data to work with.

Let me try to clear any confusion on this.

Take, for example, the simple dictionary template. It does two things: it first checks a token against a stop word list. If it finds a match it returns an empty array, which will result in the token being discarded. If no match is found in the stop word list, the process will return the same token, but with casing removed.

All the checking and case removing is done by functions, under the hood. The stop word file, however, is something that the dictionary (the instantiation) provides. The instantiation of the simple dictionary template, thus the dictionary itself, would be defined as follows:

CREATE TEXT SEARCH DICTIONARY simple (
    template = simple,
    stopwords = english
);

No need to run this SQL for PostgreSQL already comes shipped with the simple dictionary, but I wish to show you how you could create it.

First, you will see that we have to define the template, thus telling PostgreSQL which set of functions to use. Next we feed it the data it is expecting, in case of simple it only expects a stop word list.

The reason for this separation is a safe guard one. Only a database user with super user privileges can write the actual template, because this template will contain functions that, if written incorrectly, could slow down or crash the database. You need someone who knows what they are doing and not your local script kiddy who has normal user access to your part of the database.

Notice that we only give the stopwords attribute the word english instead of a full file path. This is because PostgreSQL has set a few standards in place for all dictionary types we will see in this chapter.

First, in case of a stop word list, the file must have the .stop extension.

Next, you can provide a full path to the file, anywhere on your system. However, if you do not provide a full path, PostgreSQL will search for it inside a directory called tsearch_data within PostgreSQL's portion of your system's user shared directory.

On a Debian system (using PostgreSQL 9.3) the path to this directory reads: "/usr/share/postgresql/9.3/tsearch_data".

A dictionary like the simple dictionary is one that is most of the time put at the beginning of a dictionary list to remove all the stop words before other dictionaries are being consulted. However, in all the cases where we see simple in the dictionary column of the token type list above, only this dictionary is used, meaning that only stop words are removed and all else is stripped of casing.

Creating the "simple" dictionary

Say that we wanted to setup our own simple dictionary based on the simple dictionary template, but feed it our own list of stop words. Before setting up this new dictionary, we would first have to write a stop word file.

Luckily for us, this is trivial. A stop word file is nothing more then a plain text file with one word on each line. Empty lines and trailing whitespace are ignored. We would then have to save this file with the .stop extension. Let us try just that.

Open up your editor and punch in the words "dolphin" and "the", both on their own line. Write the file out as "shisaa_stop.stop", preferably in PostgreSQL's shared directory.

Next we need to setup our dictionary. Connect to the "phrases" database from chapter one and run the following SQL:

CREATE TEXT SEARCH DICTIONARY shisaa_simple (
    template = simple,
    stopwords = shisaa_stop
);

Setting up a configuration

Now, the dictionary by itself is not very helpful. As we have seen before, we need to map it to token categories before we can actually use it for parsing. This means that we need to make our own configuration.

Let us setup an empty configuration (not based on an existing one like 'english'):

CREATE TEXT SEARCH CONFIGURATION shisaa (parser='default');

This statement will create a new configuration for us which is completely empty, it has no mappings. The argument we have to give here can be either parser or copy. With parser you define which parser to use and it will create an empty configuration. PostgreSQL has only one parser by default which is named...default. If you choose copy then you will have to provide an existing configuration name (like english) from which you would like to make a copy.

To verify that the configuration is empty, run our describe on it:

\dF+ shisaa

And marvel at its emptiness.

Now, let us add the shisaa_simple dictionary we created before:

ALTER TEXT SEARCH CONFIGURATION shisaa
    ALTER MAPPING FOR asciiword, asciihword, hword_asciipart,
                  word, hword, hword_part
    WITH shisaa_simple;

As you will see throughout this (and the next) chapter, full text extends not only the data types and functions we have available, but also extends PostgreSQL's SQL syntax with a handful of new statements. I need to note that all of these statements are not SQL standard (for SQL has no full text standard) and thus cannot be easily ported to a different database. But then again...what is this folly...who would even need a different database!

The new statements introduced here (and in the previous SQL blocks) are:

  • CREATE TEXT SEARCH DICTIONARY
  • CREATE TEXT SEARCH CONFIGURATION
  • ALTER TEXT SEARCH CONFIGURATION

Just remember that these are not part of the SQL standard (something which PostgreSQL holds very dear, in high contrast with many other databases).

Did it work? Well, describe it:

\dF+ shisaa

And get back:

 asciihword      | shisaa_simple
 asciiword       | shisaa_simple
 hword           | shisaa_simple
 hword_asciipart | shisaa_simple
 hword_part      | shisaa_simple
 word            | shisaa_simple

Perfect!

Here we mapped our fresh dictionary to the token groups "asciihword", "asciiword", "hword", "hword_asciipart", "hword_part", "word", because these will target most of a normal, English sentence.

It is time to try out this new search configuration! Punch in the same on-the-fly SQL as we had in the previous chapter, but this time with our own configuration:

SELECT to_tsvector('shisaa','The big blue elephant jumped over the crippled blue dolphin.');

And we get back:

'big':2 'blue':3,9 'crippled':8 'elephant':4 'jumped':5 'over':6

Ha! All squeaky flippers unite! The word dolphin is removed, because we defined it to be a stop word. A world as it should be.

We now have a basic full text configuration with a simple dictionary. To have a more real world full text search we will need more then just this dictionary though, we will at least need to take care of stemming.

Extending the configuration: stemming with the Snowball

Stemming, the process of reducing words to their basic form, is done by a special, dedicated kind of dictionary, the Snowball dictionary.

What?

Snowball is a very proven string processing language specially designed for stemming purposes and supports a wide range of languages. It originated from the Porter stemming algorithm and uses a natural syntax to define stemming rules.

And luckily for us, PostgreSQL has a Snowball dictionary template ready to use. This template has the Snowball stemming rules embedded for a wide variety of languages. Let us create a dictionary for our shisaa configuration, shall we?

CREATE TEXT SEARCH DICTIONARY shisaa_snowball (
    template = snowball,
    language = english
);

Again, very easy to setup. The snowball dictionary template accepts two variables to be setup. The first, mandatory one is the language you wish to support. Without this, the template does not know which of the Snowball stemming rules to take.

The next, optional one is, again, a stop word list. But...why can we feed this dictionary a stop word list? Did we not already do that with the simple dictionary?

That is correct, we did setup the simple dictionary to remove stop words for us, but we are not required to use the simple and the snowball dictionary in tandem. It is perfectly possible to map only the snowball dictionary for various token categories and ignore all other dictionaries. If you would not tell the snowball dictionary to remove stop words, it could become messy for the Snowball stemmer will try and stem all words it finds.

This stop word list can be the exact same list we fed the simple dictionary.

Also, because a snowball dictionary will try and parse all the tokens it is being fed, it is consider to be a wide dictionary. Therefor, as we have seen earlier, it is a good idea when chaining dictionaries together to put this dictionary at the end of your chain.

We now have our own version of the snowball dictionary and need to extend our configuration and map this dictionary to the desired token categories:

ALTER TEXT SEARCH CONFIGURATION shisaa
    ALTER MAPPING FOR asciiword, asciihword, hword_asciipart,
                  word, hword, hword_part
    WITH shisaa_simple, shisaa_snowball;

Notice that in the WITH clause we are now chaining the simple and the snowball dictionary together. The order is, of course, important. Describe our configuration once more:

\dF+ shisaa

And get back:

asciihword      | shisaa_simple,shisaa_snowball
asciiword       | shisaa_simple,shisaa_snowball
hword           | shisaa_simple,shisaa_snowball
hword_asciipart | shisaa_simple,shisaa_snowball
hword_part      | shisaa_simple,shisaa_snowball
word            | shisaa_simple,shisaa_snowball

Perfect, now the simple dictionary will be consulted first followed by the snowball dictionary.

Note that throughout this chapter I will chain together dictionaries in order. This will not always be the most smart or desired order, just an order to demonstrate how you can chain dictionaries.

To the test, throw a new query at it:

SELECT to_tsvector('shisaa','The big blue elephant jumped over the crippled blue dolphin.');

And get back:

'big':2 'blue':3,9 'crippled':8 'elephant':4 'jumped':5 'over':6

Nice, that is very...oh wait. Something is not correct. I am getting back exactly the same result as before. The words "crippled" and "elephant" are not stemmed at all. Why?

Well, the simple dictionary, as we defined it earlier, is setup to be a bit greedy. In its current state it will return an unmatched token as a lexeme with casing removed. It does not return NULL. And, as you know by now, NULL is needed to give other dictionaries a chance to examine the token.

So, we need to alter the simple dictionary's behavior. For this, we can use the ALTER syntax provided to us. And as it turns out, the simple dictionary template can accept one more variable: the accept variable. If this is set to false, then it will return NULL for every unmatched token. Let us alter that dictionary:

ALTER TEXT SEARCH DICTIONARY shisaa_simple ( accept = false );

Run the ts_vector query again, and look at the results:

'big':2 'blue':3,9 'crippl':8 'eleph':4 'jump':5 'over':6

That is what we were looking for, nicely stemmed results!

Extending the configuration: fun with synonyms

By now we have seen the first and the last dictionary in our control chain, but at least one more important part is missing: synonyms are not removed.

Let us extend our favorite sentence and add a few synonyms to it: "The big blue elephant, joined by its enormous blue mammoth friend, jumped over the crippled blue dolphin while smiling at the orca."

Still perfectly possible.

In the light of (cue dark en deep Batman voice) "science" (end Batman voice), let us first see what we get when we run it through our current configuration:

'at':20 'big':2 'blue':3,9,16 'by':6 'crippl':15 'eleph':4 'enorm':8 'friend':11 'it':7 'join':5 'jump':12 'mammoth':10 'orca':22 'over':13 'smile':19 'while':18

That is one big result set. Maybe we should cut the blue dolphin a little bit of slack and feed a real stop word list to our simple dictionary before continuing by altering our dictionary:

ALTER TEXT SEARCH DICTIONARY shisaa_simple ( stopwords = english );

As you see you can simply use the same ALTER syntax as before. The "english" here refers to the shipped "english.stop" stop word list.

Querying again, we will get back a better, short list (including our Dolphin friend):

'big':2 'blue':3,9,16 'crippl':15 'dolphin':17 'eleph':4 'enorm':8 'friend':11 'join':5 'jump':12 'mammoth':10 'orca':22 'smile':19

Now we would like to reduce this result even further by compacting synonyms into one lexeme.

Enter the synonym dictionary template.

This template requires you to have a so-called "synonym" file; A file containing lists of words with the same meaning. For the sake of learning, let us create our own synonym file. This file has to end with the .syn extension.

Open up your editor again and write out a file called "shisaa_syn.syn" with the following contents:

big enormous
elephant mammoth
dolphin orca

And let us setup the dictionary:

CREATE TEXT SEARCH DICTIONARY shisaa_synonym (
    template = synonym,
    synonyms = shisaa_syn
);

And add the mapping for it:

ALTER TEXT SEARCH CONFIGURATION shisaa
    ALTER MAPPING FOR asciiword, asciihword, hword_asciipart,
                  word, hword, hword_part
    WITH shisaa_simple, shisaa_synonym, shisaa_snowball;

Okay, time to test our big string again and see the results:

'blue':3,9,16 'crippl':15 'enorm':8 'enormous':2 'friend':11 'join':5 'jump':12 'mammoth':4,10 'orca':17,22 'smile':19

Very neat. The words "elephant", "big" and "dolphin" are now removed and only their synonyms are kept. Also notice that both "mammoth" and "orca" have two pointers each, one for every synonym.

But look at the words 'enorm' and 'enormous', why is this happening?

If you look at the pointers, you see that enormous points to the second word in the string, being big, while enorm points to the original enormous word. The reason why this is happening is because our synonym dictionary has priority over our snowball one. The synonym dictionary emits a lexeme as a synonym for big, being enormous, simply because we told it to do so in our synonym file. Now, because it emits a lexeme, the original token, big, is not available anymore for the rest of the dictionary chain.

The token enormous itself has no synonym because we did not define it in our synonym file. It is ignored by the synonym dictionary and passed over to the snowball dictionary which then stems the token into a lexeme resulting in enorm.

If you wish to prevent this from happening, you could add a self pointing line to your synonym list:

enormous enormous

Now load in the file on disk to pull the changes into PostgreSQL:

 ALTER TEXT SEARCH DICTIONARY shisaa_synonym (synonyms=shisaa_syn);

And run the query again, the result should now read:

'blue':3,9,16 'crippl':15 'enormous':2,8 'friend':11 'join':5 'jump':12 'mammoth':4,10 'orca':17,22 'smile':19

Now enorm will be removed and both big and enormous are cast to the same lexeme.

PostgreSQL does not ship a synonym list, so you will have to compile your own just like we did above but hopefully a little bit more useful

Extending the configuration: phrasing with a Thesaurus

Next up is the thesaurus dictionary, which is quite close to the synonym dictionary, with one exception: phrases.

A thesaurus dictionary is used to recognize phrases and convert them into lexemes with the same meaning. Again, this dictionary relies on a file containing the phrase conversions. This time, the file has the .ths extension.

Open up your editor and write out a file called "shisaa_thesaurus.ths" with the following contents:

big blue elephant : PostgreSQL
crippled blue dolphin : MySQL

Before we can create the dictionary, there is one more required variable we have to set, the subdictionary the thesaurus dictionary can use. This subdictionary will be another dictionary you have defined before. Most of the time a stemmer is fed to this variable to let the thesaurus stem the input before comparing it with its thesaurus file.

So let us feed it our snowball dictionary and set it up:

CREATE TEXT SEARCH DICTIONARY shisaa_thesaurus (
    TEMPLATE = thesaurus,
    DICTFILE = shisaa_thesaurus,
    DICTIONARY = shisaa_snowball
);

Map it:

ALTER TEXT SEARCH CONFIGURATION shisaa
    ALTER MAPPING FOR asciiword, asciihword, hword_asciipart,
                  word, hword, hword_part
    WITH shisaa_simple, shisaa_thesaurus, shisaa_snowball;

Notice that I took out the synonym dictionary. If we chain up to many dictionaries, the results might turn out to be undesirable in our demonstration use case.

Querying will result in the following tsvector:

'blue':7 'enorm':6 'friend':9 'join':3 'jump':10 'mammoth':8 'mysql':13 'orca':18 'postgresql':2 'smile':15

That is quite awesome, it now recognizes "big blue elephant" as PostgreSQL and "crippled blue dolphin" as MySQL. We have created a pun-aware full text search configuration!

As you can see, both the "MySQL" and "PostgreSQL" lexemes have one pointer each, pointing to the first word of the substring that got converted.

Extending the configuration a last time: morphing with Ispell

Okay, we are almost at the end of the dictionary templates that PostgreSQL supports.

This last one is a fun one too. Many Unix and Linux systems come shipped with a spell checker called Ispell or with the more modern variant called HunSpell. Besides your average spell checking, these dictionaries are very good at morphological lookups, meaning that they can link all different writing structures of words together.

A synonym or thesaurus dictionary would not catch these, unless explicitly set with a huge amount of lines in the .syn or .ths files, which is error prone and inelegant. The Ispell or Hunspell dictionaries will capture these and try to make them into one lexeme.

Before setting up the dictionary, we first need to make sure that we have the Ispell or Hunspell dictionary files for the language we wish to support. Normally you would want to download these files from the official OpenOffice page. These pages, however, seem to be confusing and the correct files very hard to find. I have found the following page of great help to get the files you need for your desired language . Download the files for your desired language and place the .dict and the .affix files into the PostgreSQL shared directory.

For now, let us just take the basic english dict and affix files (named both en_us and already shipped with PostgreSQL) and feed them to the configuration:

CREATE TEXT SEARCH DICTIONARY shisaa_ispell (
    template = ispell,
    DictFile = en_us,
    AffFile = en_us,
    StopWords = english
);

And chain it:

ALTER TEXT SEARCH CONFIGURATION shisaa
    ALTER MAPPING FOR asciiword, asciihword, hword_asciipart,
                  word, hword, hword_part
    WITH shisaa_simple, shisaa_ispell, shisaa_snowball;

Notice again I took out the thesaurus dictionary, not to pile up too many dictionaries at once.

Query it once more, and look at what we get back:

'big':2 'blue':3,9,16 'cripple':15 'dolphin':17 'elephant':4 'enormous':8 'friend':11 'join':5 'joined':5 'jump':12 'mammoth':10 'orca':22 'smile':19 'smiling':19

Hmm, interesting. Notice that we now got more lexemes than before, smile and smiling for example, and join and joined. Also, both these cases have the same pointer. Why is that?

What is happening here is a feature of the Ispell dictionary called morphology, or as we seen above, morphological lookups. One of the reasons why Ispell is such a powerful dictionary is because it can recognize and act upon the structure of a word.

In our case, Ispell recognizes joined (or smiling) and emits an array of two lexemes, the original token converted to a lexeme and the stemmed version of the token.

This concludes all the dictionaries that PostgreSQL ships with by default and the ones you will most likely ever need. What is next?

Debugging

Now that you have a good understanding of how to build your own configuration and setup your own dictionaries, I would like to introduce a few new functions that can come in handy when your configuration would produce seemingly strange results.

ts_debug()

The first function I want show you is a very handy one that is built to test your whole full text configuration. It helps you keep your mental condition to just mildly insane, so to speak.

The function ts_debug() accepts a configuration and a string of text you wish to test. As a result you will get back a set that contains an overview of how the parser chopped your string into tokens, which category it picked for each token, which dictionary was consulted and which lexeme(s) where emitted. Oh boy, this is too much fun, let us just try it out!

Feed our original pun string and let us test the current shisaa configuration:

SELECT ts_debug('shisaa','The big blue elephant jumped over the crippled blue dolphin.');

Hmm, that may not be very readable, rather use the wildcard selector and a FROM clause to include column names into our result set (one of the few times you may use this selector without getting smacked):

SELECT * FROM ts_debug('shisaa','The big blue elephant jumped over the crippled blue dolphin.');

Which will result in the following, huge set:

  alias   |   description   |  token   |                 dictionaries                  |  dictionary   |  lexemes   
-----------+-----------------+----------+-----------------------------------------------+---------------+------------
asciiword | Word, all ASCII | The      | {shisaa_simple,shisaa_ispell,shisaa_snowball} | shisaa_simple | {}
blank     | Space symbols   |          | {}                                            |               | 
asciiword | Word, all ASCII | big      | {shisaa_simple,shisaa_ispell,shisaa_snowball} | shisaa_ispell | {big}
blank     | Space symbols   |          | {}                                            |               | 
asciiword | Word, all ASCII | blue     | {shisaa_simple,shisaa_ispell,shisaa_snowball} | shisaa_ispell | {blue}
blank     | Space symbols   |          | {}                                            |               | 
asciiword | Word, all ASCII | elephant | {shisaa_simple,shisaa_ispell,shisaa_snowball} | shisaa_ispell | {elephant}
blank     | Space symbols   |          | {}                                            |               | 
asciiword | Word, all ASCII | jumped   | {shisaa_simple,shisaa_ispell,shisaa_snowball} | shisaa_ispell | {jump}
blank     | Space symbols   |          | {}                                            |               | 
asciiword | Word, all ASCII | over     | {shisaa_simple,shisaa_ispell,shisaa_snowball} | shisaa_simple | {}
blank     | Space symbols   |          | {}                                            |               | 
asciiword | Word, all ASCII | the      | {shisaa_simple,shisaa_ispell,shisaa_snowball} | shisaa_simple | {}
blank     | Space symbols   |          | {}                                            |               | 
asciiword | Word, all ASCII | crippled | {shisaa_simple,shisaa_ispell,shisaa_snowball} | shisaa_ispell | {cripple}
blank     | Space symbols   |          | {}                                            |               | 
asciiword | Word, all ASCII | blue     | {shisaa_simple,shisaa_ispell,shisaa_snowball} | shisaa_ispell | {blue}
blank     | Space symbols   |          | {}                                            |               | 
asciiword | Word, all ASCII | dolphin  | {shisaa_simple,shisaa_ispell,shisaa_snowball} | shisaa_ispell | {dolphin}
blank     | Space symbols   | .        | {}                                            |               |

You now have a complete overview of the flow from string to vector of lexemes. Let me go over some interesting facts of this result set.

First, notice how the tokens the and over got removed by the simple dictionary. They where a hit in the stop word list, so the dictionary returned an empty array.

Next you see the alias blank between each asciiword. Blank is a category used for spaces or punctuation. A space and a . (full stop) is considered a token, but is stripped out by the parser itself for it has no value in this context.

And last, see that our snowball dictionary was never consulted. This means that, in this string, the shisaa_ispell gobbled all the lexemes that shisaa_simple threw at it.

ts_lexize()

The second function is ts_lexize(). This little helper lets you test different parts of your whole setup. Take the unexpected result of our last dictionary, where we got back multiple lexemes. As it turned out it is normal behavior, but you may want to verify that the result is coming from the dictionary and not from a side effect of how you chained your dictionaries together.

To test our single, shisaa_ispell dictionary, we could feed it to this new function, together with one token we wish to test:

SELECT ts_lexize('shisaa_ispell','joined');

This will return:

{joined,join}

Same as we had before, but now we know, for sure, that it is a feature of our Ispell dictionary. Notice that I stressed the fact that you can only feed this function one token, not a string of text and not multiple tokens.

You can use this function to test all your dictionaries individually, one token at a time.

Phew, that was a lot to take in for we covered a lot of ground here today. You can turn the lights back high and go get some fresh air. In the next chapter, I will round up this introduction by introducing you to the following, new material:

  • Ranking search results
  • Highlighting words inside search results
  • Creating special full text search indexes
  • Setting up update triggers for tsvector records

And as always...thanks for reading!