FTS Templates

fts_templates FTS Templates#

What is an FTS Template?#

An FTS template is the blueprint for a dictionary.

  • It defines how a dictionary behaves

  • It does not contain language rules itself

  • Dictionaries are instances created from templates

Think of it like this:

Template = class

Dictionary = object created from that class

Where templates fit in the FTS pipeline#

Text
↓
Parser
↓
Token types
↓
Configuration
↓
Dictionary (built from a template)
↓
tsvector

  • Parser splits text

  • Configuration decides which dictionary to use

  • Dictionary processes tokens

  • Template defines how that dictionary works internally

Built-in FTS templates#

You’ll see these under Schemas → public → FTS Templates:

  1. simple

What it does

  • Accepts tokens as-is

  • Lowercases text

  • No stemming

  • No stop-word removal

Use when

  • You want exact word matching

  • You don’t want linguistic changes

  1. snowball

What it does

  • Uses Snowball stemming algorithms

  • Reduces words to a root form

  • Removes stop words

Use when

  • You want linguistic search

  • “running”, “ran”, “runs” → same root

  1. ispell

What it does

  • Uses dictionary + affix files

  • Very accurate linguistic processing

  • Heavier to manage

Use when

  • You need strict dictionary-based normalization

  1. thesaurus

What it does

  • Maps words/phrases to synonyms

  • Expands search meaning

Use when

  • You want synonym-aware search

  • Example: “car” → “automobile”

Templates vs Dictionaries#

Concept

What it is

Template

Processing engine

Dictionary

Configured instance of a template

Example:

CREATE TEXT SEARCH DICTIONARY my_simple_dict (
    TEMPLATE = simple
);
Here:

  • simple = template

  • my_simple_dict = dictionary

Why templates matter#

Templates define:

  • Whether stemming is applied

  • Whether stop words are removed

  • Whether normalization happens

  • Whether external files are used

You cannot change behavior directly in a dictionary → You choose the correct template

Why you almost never create templates yourself#

Templates:

  • Are low-level

  • Are implemented in C

  • Are part of PostgreSQL internals

In real systems:

  • âś… You create dictionaries

  • ❌ You do not create templates

How pgAdmin shows templates

pgAdmin lists:

  • Template name

  • Underlying implementation

  • Whether it supports lexeme filtering

This mirrors:

SELECT * FROM pg_ts_template;

Practical example (template → dictionary → config)

-- Dictionary from template
CREATE TEXT SEARCH DICTIONARY my_dict (
    TEMPLATE = simple
);

-- Configuration using the dictionary
ALTER TEXT SEARCH CONFIGURATION pg_catalog.english
    ALTER MAPPING FOR word
    WITH my_dict;
Summary:

FTS templates define how dictionaries work; dictionaries do the actual text processing.

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