Add Troubleshooting page (#394)

This is based on https://www.clear-code.com/blog/2024/10/25/check-points-use-pgroonga-index.html .

index.md

@@ -48,6 +48,8 @@ PostgreSQL supports full text search against languages that use only alphabet an
   * [How to](how-to/): It describes about useful information for specific situations.
 
   * [Reference](reference/): It describes details for each features such as options, functions and operators.
+
+  * [Troubleshooting](troubleshooting/): It describes how to fix troubles.
 
   * [Community](community/): It introduces about PGroonga community.
 

troubleshooting/index.md

@@ -0,0 +1,77 @@
+---
+title: Troubleshooting
+---
+# Troubleshooting
+
+Many PGroonga search issues stem from improper index setup. This troubleshooting guide addresses these problems.
+
+The following sections use a Q&A format to explain these issues in detail.
+
+## Q: I created a PGroonga index, but my searches are still slow.
+
+A: When a search is slow, it often means PostgreSQL is using a sequential scan instead of the PGroonga index. To check this, run your query with `EXPLAIN ANALYZE`. 
+
+If you see "Seq Scan" in the output, it indicates the system is not utilizing the index. The goal is to see "Index Scan using pgrn_content_index" or similar, indicating the index is being used.
+
+Check the table configuration to confirm that PGroonga indexes are properly set.
+
+<details>
+
+<summary>To confirm whether a sequential search is being executed</summary>
+
+We use a following table structure as an example.
+
+To ensure the search is definitely sequential in this example, no indexes or primary keys have been set.
+
+```sql
+CREATE TABLE memos (
+  title text,
+  content text
+);
+
+INSERT INTO memos VALUES ('PostgreSQL', 'PostgreSQL is an RDBMS.');
+INSERT INTO memos VALUES ('Groonga', 'Groonga is a super-fast full-text search engine.');
+INSERT INTO memos VALUES ('PGroonga', 'PGroonga is an extension that brings super-fast full-text search to PostgreSQL.');
+```
+
+The query we are examining is as follows:
+
+```sql
+SELECT * FROM memos WHERE content &@~ 'PostgreSQL';
+```
+
+Now, let's confirm whether the query is using a sequential search.
+
+As mentioned earlier, we'll use `EXPLAIN ANALYZE` to check.
+
+```sql
+EXPLAIN ANALYZE SELECT * FROM memos WHERE content &@~ 'PostgreSQL';
+--                                              QUERY PLAN                                              
+-- -----------------------------------------------------------------------------------------------------
+--  Seq Scan on memos  (cost=0.00..678.80 rows=1 width=64) (actual time=2.803..4.664 rows=2 loops=1)
+--    Filter: (content &@~ 'PostgreSQL'::text)
+--    Rows Removed by Filter: 1
+--  Planning Time: 0.113 ms
+--  Execution Time: 4.731 ms
+-- (5 rows)
+```
+
+The result is as shown above.
+
+In the case of a sequential search, "Seq Scan" will be displayed.
+
+Our goal here is to transform this "Seq Scan" into "Index Scan using #{PGroonga index name}" as shown below.
+
+```sql
+EXPLAIN ANALYZE SELECT * FROM memos WHERE content &@~ 'PostgreSQL';
+--                                                           QUERY PLAN                                                          
+-- ------------------------------------------------------------------------------------------------------------------------------
+--  Index Scan using pgrn_content_index on memos  (cost=0.00..4.02 rows=1 width=64) (actual time=0.778..0.782 rows=2 loops=1)
+--    Index Cond: (content &@~ 'PostgreSQL'::text)
+--  Planning Time: 0.835 ms
+--  Execution Time: 1.002 ms
+-- (4 rows)
+```
+
+</details>
+