Constraints on Index Use

The following is an example of index use:

mogdb=# create table table1 (c_int int,c_bigint bigint,c_varchar varchar,c_text text) with(orientation=row);

mogdb=# create text search configuration ts_conf_1(parser=POUND);
mogdb=# create text search configuration ts_conf_2(parser=POUND) with(split_flag='%');

mogdb=# set default_text_search_config='ts_conf_1';
mogdb=# create index idx1 on table1 using gin(to_tsvector(c_text));

mogdb=# set default_text_search_config='ts_conf_2';
mogdb=# create index idx2 on table1 using gin(to_tsvector(c_text));

mogdb=# select c_varchar,to_tsvector(c_varchar) from table1 where to_tsvector(c_text) @@ plainto_tsquery('￥#@……&**')   and to_tsvector(c_text) @@ plainto_tsquery('某公司 ')   and c_varchar is not null order by 1 desc limit 3;

In this example, table1 has two GIN indexes created on the same column c_text, idx1 and idx2, but these two indexes are created under different settings of default_text_search_config. Differences between this example and the scenario where one table has common indexes created on the same column are as follows:

• GIN indexes use different parsers (that is, different delimiters). In this case, the index data of idx1 is different from that of idx2.
• In the specified scenario, the index data of multiple common indexes created on the same column is the same.

As a result, using idx1 and idx2 for the same query returns different results.

Constraints

Still use the above example. When:

• Multiple GIN indexes are created on the same column of the same table.

• The GIN indexes use different parsers (that is, different delimiters).

• The column is used in a query, and an index scan is used in the execution plan.

  To avoid different query results caused by different GIN indexes, ensure that only one GIN index is available on a column of the physical table.