ApsaraDB RDS:Use the pg_bigm extension to perform fuzzy match-based queries

Comparison between the pg_bigm extension and the pg_trgm extension

The pg_trgm extension is also provided by ApsaraDB RDS for PostgreSQL. The pg_trgm extension uses a 3-gram model to perform full-text search. The pg_bigm extension is developed based on the pg_trgm extension. The following table describes the differences between the two extensions.

Usage notes

• The length of the column on which you create a GIN index cannot exceed 107,374,180 bytes, which are approximately 102 MB.

• If the data in your RDS instance is not encoded in the ASCII format, we recommend that you change the encoding format to UTF8.

  Note

  You can run the select pg_encoding_to_char(encoding) from pg_database where datname = current_database(); command to query the encoding format of your RDS instance.

Basic operations

• Enable the pg_bigm extension.

  postgres=> create extension pg_bigm;
  CREATE EXTENSION

• Create a GIN index.

  postgres=> CREATE TABLE pg_tools (tool text, description text);
  CREATE TABLE
  postgres=> INSERT INTO pg_tools VALUES ('pg_hint_plan', 'Tool that allows a user to specify an optimizer HINT to PostgreSQL');
  INSERT 0 1
  postgres=> INSERT INTO pg_tools VALUES ('pg_dbms_stats', 'Tool that allows a user to stabilize planner statistics in PostgreSQL');
  INSERT 0 1
  postgres=> INSERT INTO pg_tools VALUES ('pg_bigm', 'Tool that provides 2-gram full text search capability in PostgreSQL');
  INSERT 0 1
  postgres=> INSERT INTO pg_tools VALUES ('pg_trgm', 'Tool that provides 3-gram full text search capability in PostgreSQL');
  INSERT 0 1
  postgres=> CREATE INDEX pg_tools_idx ON pg_tools USING gin (description gin_bigm_ops);
  CREATE INDEX
  postgres=> CREATE INDEX pg_tools_multi_idx ON pg_tools USING gin (tool gin_bigm_ops, description gin_bigm_ops) WITH (FASTUPDATE = off);
  CREATE INDEX

• Run a full-text search query.

  postgres=> SELECT * FROM pg_tools WHERE description LIKE '%search%';
    tool   |                             description
  ---------+---------------------------------------------------------------------
   pg_bigm | Tool that provides 2-gram full text search capability in PostgreSQL
   pg_trgm | Tool that provides 3-gram full text search capability in PostgreSQL
  (2 rows)

• Use the =% operator to run a similarity search query.

  postgres=> SET pg_bigm.similarity_limit TO 0.2;
  SET
  postgres=> SELECT tool FROM pg_tools WHERE tool =% 'bigm';
    tool
  ---------
   pg_bigm
   pg_trgm
  (2 rows)

• Disable the pg_bigm extension.

  postgres=> drop extension pg_bigm;
  DROP EXTENSION

Basic functions

• likequery

  • Purpose: This function is used to generate a string that can be identified based on the LIKE keyword.

  • Request parameters: This function contains one request parameter. The data type for this parameter is STRING.

  • Return value: This function returns a string that can be identified based on the LIKE keyword.

  • Implementation:

    • Add a percent sign (%) preceding and following the keyword.

    • Use a backward slash (\) to escape the percent sign (%).

  • Example:

    postgres=> SELECT likequery('pg_bigm has improved the full text search performance by 200%');
                                 likequery
    -------------------------------------------------------------------
     %pg\_bigm has improved the full text search performance by 200\%%
    (1 row)
    
    postgres=> SELECT * FROM pg_tools WHERE description LIKE likequery('search');
      tool   |                             description
    ---------+---------------------------------------------------------------------
     pg_bigm | Tool that provides 2-gram full text search capability in PostgreSQL
     pg_trgm | Tool that provides 3-gram full text search capability in PostgreSQL
    (2 rows)

• show_bigm

  • Purpose: This function is used to obtain all 2-gram elements of a string.

  • Request parameters: This function contains one request parameter. The data type for this parameter is STRING.

  • Return value: This parameter returns an array that consists of all 2-gram elements of a string.

  • Implementation:

    • Add a space preceding and following the string.

    • Identify all 2-gram elements in the string.

  • Example:

    postgres=> SELECT show_bigm('full text search');
                                show_bigm
    ------------------------------------------------------------------
     {" f"," s"," t",ar,ch,ea,ex,fu,"h ","l ",ll,rc,se,"t ",te,ul,xt}
    (1 row)

• bigm_similarity

  • Purpose: This function is used to obtain the similarity between two strings.

  • Request parameters: This function contains two request parameters. The data types for these parameters are STRING.

  • Return value: This function returns a floating-point number, which indicates the similarity between the two strings.

  • Implementation:

    • Identify the 2-gram elements that are included in both of the two strings.

    • The return value is within the range of 0 to 1. The value 0 indicates that the two strings are different. The value 1 indicates that the two strings are the same.

    Note

    • This function adds a space preceding and following each string. Therefore, the similarity between the ABC string and the B string is 0, and the similarity between the ABC string and the A string is 0.25.

    • This function distinguishes between uppercase letters and lowercase letters. For example, this function determines that the similarity between the ABC string and the abc string is 0.

  • Example:

    postgres=> SELECT bigm_similarity('full text search', 'text similarity search');
     bigm_similarity
    -----------------
           0.5714286
    (1 row)
    
    postgres=> SELECT bigm_similarity('ABC', 'A');
     bigm_similarity
    -----------------
                0.25
    (1 row)
    
    postgres=> SELECT bigm_similarity('ABC', 'B');
     bigm_similarity
    -----------------
                   0
    (1 row)
    
    postgres=> SELECT bigm_similarity('ABC', 'abc');
     bigm_similarity
    -----------------
                   0
    (1 row)

• pg_gin_pending_stats

  • Purpose: This function is used to obtain the number of pages and the number of tuples in the pending list of a GIN index.

  • Request parameters: This function contains one parameter. This parameter specifies the name or OID of the GIN index.

  • Return value: This function returns two values: the number of pages and the number of tuples in the pending list of the GIN index.

    Note

    If you set the FASTUPDATE parameter to False for a GIN index, the GIN index does not have a pending list. In this case, this function returns two values, 0 and 0.

  • Example:

    postgres=> SELECT * FROM pg_gin_pending_stats('pg_tools_idx');
     pages | tuples
    -------+--------
         0 |      0
    (1 row)

Behavior control

• pg_bigm.last_update

  This parameter indicates the date on which the most recent update was made to the pg_bigm extension. You can only view this parameter. You cannot change the value of this parameter.

  Example:

  SHOW pg_bigm.last_update;

• pg_bigm.enable_recheck

  This parameter specifies whether to perform a recheck.

  Note

  We recommend that you retain the default value ON. This way, you can obtain accurate query results.

  Example:

  postgres=> CREATE TABLE tbl (doc text);
  CREATE TABLE
  postgres=> INSERT INTO tbl VALUES('He is awaiting trial');
  INSERT 0 1
  postgres=> INSERT INTO tbl VALUES('It was a trivial mistake');
  INSERT 0 1
  postgres=> CREATE INDEX tbl_idx ON tbl USING gin (doc gin_bigm_ops);
  CREATE INDEX
  postgres=> SET enable_seqscan TO off;
  SET
  postgres=> EXPLAIN ANALYZE SELECT * FROM tbl WHERE doc LIKE likequery('trial');
                                                     QUERY PLAN
  -----------------------------------------------------------------------------------------------------------------
   Bitmap Heap Scan on tbl  (cost=20.00..24.01 rows=1 width=32) (actual time=0.020..0.021 rows=1 loops=1)
     Recheck Cond: (doc ~~ '%trial%'::text)
     Rows Removed by Index Recheck: 1
     Heap Blocks: exact=1
     ->  Bitmap Index Scan on tbl_idx  (cost=0.00..20.00 rows=1 width=0) (actual time=0.013..0.013 rows=2 loops=1)
           Index Cond: (doc ~~ '%trial%'::text)
   Planning Time: 0.117 ms
   Execution Time: 0.043 ms
  (8 rows)
  
  postgres=>
  postgres=> SELECT * FROM tbl WHERE doc LIKE likequery('trial');
           doc
  ----------------------
   He is awaiting trial
  (1 row)
  
  postgres=> SET pg_bigm.enable_recheck = off;
  SET
  postgres=> SELECT * FROM tbl WHERE doc LIKE likequery('trial');
             doc
  --------------------------
   He is awaiting trial
   It was a trivial mistake
  (2 rows)

• pg_bigm.gin_key_limit

  This parameter specifies the maximum number of 2-gram elements that can be used to perform a full-text search query. The default value is 0, which indicates that all 2-gram elements are used.

  Note

  If query performance decreases due to the use of all 2-gram elements, you can decrease the value of this parameter.

• pg_bigm.similarity_limit

  This parameter specifies the threshold for similarity. The tuples between which the similarity exceeds the specified threshold are returned as similarity search results.