FTS3 and FTS4 are SQLite virtual table modules that allows users to perform full-text searches on a set of documents.
The basic idea is to have a virtual FTS table for each register table.
The virtual table holds the values that are used for searching and sorting as well as an identifier to link the register table.
Prerequisite
Always consider explain query plan
to determine whether your queries will be slow.
For Example:
Code Block | ||
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explain query plan select * from household where FWHOHFNAME LIKE '%abc%' |
Returns
selectid | order | from | detail |
---|---|---|---|
0 | 0 | 0 | SCAN TABLE household |
Full table scans are really slow, especially when querying a large data set. Consider optimizing such queries.
Adopting FTS search
*Application.java → Add the register table names
Code Block language java theme Eclipse private String[] getFtsTables(){ String[] ftsTables = { "table1" ... }; return ftsTables; }
Add search and sort fields
Code Block language java theme Eclipse private String[] getFtsSearchFields(String tableName){ if(tableName.equals("table1")){ String[] ftsSearchFields = { "SearchField1" ... }; return ftsSearchFields; } ... } private String[] getFtsSortFields(String tableName){ if(tableName.equals("table1")) { String[] sortFields = {"SortField1" ... }; return sortFields; } ... return null; }
Add custom relational Ids if they exist.
These are foreign keys apart from relationalId or relational_id fields that store a unique identifier for the related table e.g. childId or parentId
Code Block language java theme Eclipse private String getFtsCustomRelationalId(String tableName){ if(tableName.equals("table1")){ String customRelationalId = "parentId1"; return customRelationalId; } ... return null; }
Add condition fields if the query has a main condition
For Example:
Code Block language java theme Eclipse queryBUilder.mainCondition(" Field1 is not null ");
Code Block language java theme Eclipse private String[] getFtsMainConditions(String tableName){ if(tableName.equals("table1")) { String[] mainConditions = {"Field1"}; return mainConditions; } ... }
In *Application.onCreate(), update the global context instance i.e. update context.commonFtsObject
Code Block language java theme Eclipse context.updateCommonFtsObject(createCommonFtsObject());
Code Block language java theme Eclipse private CommonFtsObject createCommonFtsObject(){ CommonFtsObject commonFtsObject = new CommonFtsObject(getFtsTables()); for(String ftsTable: commonFtsObject.getTables()){ commonFtsObject.updateSearchFields(ftsTable, getFtsSearchFields(ftsTable)); commonFtsObject.updateSortFields(ftsTable, getFtsSortFields(ftsTable)); commonFtsObject.updateMainConditions(ftsTable, getFtsMainConditions(ftsTable)); commonFtsObject.updateCustomRelationalId(ftsTable, getFtsCustomRelationalId(ftsTable)); } return commonFtsObject; }
In org.ei.opensrp.commonregistry.CommonRepository#onCreate, id and relationalid indexes are created for each common repository table.
Code Block language java theme Eclipse if(StringUtils.isNotBlank(common_ID_INDEX_SQL)) { database.execSQL(common_ID_INDEX_SQL); } if(StringUtils.isNotBlank(common_Relational_ID_INDEX_SQL)) { database.execSQL(common_Relational_ID_INDEX_SQL); } if(StringUtils.isNotBlank(common_Custom_Relational_ID_INDEX_SQL)) { database.execSQL(common_Custom_Relational_ID_INDEX_SQL); }
In org.ei.opensrp.Context.java, commonFtsObject is passed as an argument when initializing the Repository class.
Code Block language java theme Eclipse if(commonFtsObject != null){ repository = new Repository(this.applicationContext, session(), this.commonFtsObject, drishtireposotoryarray); }
Then the virtual tables are created in org.ei.opensrp.repository.Repository#onCreate just after the database has been created.
Code Block language java theme Eclipse if(this.commonFtsObject != null) { for (String ftsTable: commonFtsObject.getTables()) { Set<String> searchColumns = new LinkedHashSet<String>(); searchColumns.add(CommonFtsObject.idColumn); searchColumns.add(CommonFtsObject.relationalIdColumn); searchColumns.add(CommonFtsObject.phraseColumnName); String[] mainConditions = this.commonFtsObject.getMainConditions(ftsTable); if(mainConditions != null) for (String mainCondition : mainConditions) { searchColumns.add(mainCondition); } String[] sortFields = this.commonFtsObject.getSortFields(ftsTable); if(sortFields != null) for (String sortValue : sortFields) { searchColumns.add(sortValue); } String joinedSearchColumns = StringUtils.join(searchColumns, ","); String searchSql = "create virtual table " + CommonFtsObject.searchTableName(ftsTable) + " using fts4 (" + joinedSearchColumns + ");"; database.execSQL(searchSql); } }
These virtual tables are updated after a form submission is saved.
For Example, in FormSubmissionService.java:
Code Block language java theme Eclipse ziggyService.saveForm(getParams(submission), submission.instance()); // Update FTS Tables updateFTSsearch(submission);
- Update filterandSortExecute() and CountExecute() methods to use FTS search
Perform search/sort query in the virtual table(s) and return the reference ids
Virtual table scans are optimized and therefore faster than normal full table scans.
Code Block language sql title Explain query plan explain query plan select object_id from household_search where phrase match 'abc*'
Returns
selectid order from detail 0 0 0 SCAN TABLE household_search VIRTUAL TABLE INDEX 4: Use the ids returned in Step 9(a) above to filter the main query
Since id fields are indexed, Step 6, full table scans are replaced with search table using index.
Code Block language sql title Explain query plan explain query plan select * from household where id in (select object_id from household_search where phrase match 'abc*')
Returns
selectid order from detail 0 0 0 SEARCH TABLE household USING INDEX sqlite_autoindex_household_1 (id=?) 0 0 0 EXECUTE LIST SUBQUERY 1 0
0 0 SCAN TABLE household_search VIRTUAL TABLE INDEX 4:
Step 9 a) and b) in codeCode Block language java theme Eclipse title Using FTS // Get the virtual table query String sql = sqb.searchQueryFts(tablename, joinTable, mainCondition, filters, Sortqueries, currentlimit, currentoffset); // Get the reference ids List<String> ids = commonRepository.findSearchIds(sql); // Update the main query by adding the reference ids in the where clause currentquery = sqb.toStringFts(ids, tablename + "." + CommonRepository.ID_COLUMN, Sortqueries); query = sqb.Endquery(currentquery);
FTS Table Structure
Code Block | ||
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PRAGMA table_info('household_search'); |
Returns
cid | name | type | not null |
| pk |
---|---|---|---|---|---|
0 | object_id | 0 | 0 | ||
1 | object_relational_id | 0 | 0 | ||
2 | phrase | 0 | 0 | ||
3 | SortField1 | 0 | 0 | ||
4 | SortField2 | 0 | 0 | ||
5 | SortField3 | 0 | 0 |
object_id → reference id i.e. household.id
object_relational_id → reference relational id i.e. household.relationalid. Useful when query has a join between two register tables.
phrase → search term, contains all the searchable values separated by ' | ' e.g. " searchFieldValue1 | searchFieldValue2 | searchFieldValue3 "
sort field → sort value
Facts
- Queries are faster when indexes are used.
- Indexes are ignored when SQLite's LIKE-keyword based query is used.
- Virtual tables require more disk space compared to regular tables.
- It takes more time to insert a record in a virtual table.
- Always consider
explain query plan
to determine whether your queries aren’t going to work slowly. - FTS does not support suffix search, only prefix.
Links
https://www.sqlite.org/fts3.html
https://lab.getbase.com/killer-search-on-android-part-1
http://stackoverflow.com/questions/29815248/full-text-search-example-in-android