How to use SQLite's extension mechanism?

SQLite's extension mechanism allows developers to add custom functionality:

1. Extension Overview

   • SQLite supports adding custom functions, aggregate functions, virtual tables, etc. through extensions
   • Extensions can be dynamically loaded or statically linked
   • Extensions are written in C/C++ and follow SQLite's API specifications

2. Loading Extensions

   sql
   -- Enable extension loading
   PRAGMA load_extension = ON;
   
   -- Load extension
   SELECT load_extension('extension_path');
   
   -- Load built-in extensions (like FTS5)
   SELECT load_extension('sqlite3ext');

3. Creating Scalar Functions

   c
   // C code example: Create custom function
   #include <sqlite3ext.h>
   SQLITE_EXTENSION_INIT1
   
   static void my_function(sqlite3_context *context, int argc, sqlite3_value **argv) {
       if (argc != 1) {
           sqlite3_result_error(context, "Invalid argument count", -1);
           return;
       }
       
       const char *text = (const char *)sqlite3_value_text(argv[0]);
       // Processing logic...
       sqlite3_result_text(context, "result", -1, SQLITE_TRANSIENT);
   }
   
   #ifdef _WIN32
   __declspec(dllexport)
   #endif
   int sqlite3_myextension_init(sqlite3 *db, char **pzErrMsg, const sqlite3_api_routines *pApi) {
       SQLITE_EXTENSION_INIT2(pApi);
       sqlite3_create_function(db, "my_function", 1, SQLITE_UTF8, NULL, my_function, NULL, NULL);
       return SQLITE_OK;
   }

4. Creating Aggregate Functions

   c
   // C code example: Create custom aggregate function
   typedef struct {
       double sum;
       int count;
   } AvgCtx;
   
   static void avg_step(sqlite3_context *context, int argc, sqlite3_value **argv) {
       AvgCtx *p = (AvgCtx *)sqlite3_aggregate_context(context, sizeof(AvgCtx));
       if (p->count == 0) {
           p->sum = 0;
       }
       p->sum += sqlite3_value_double(argv[0]);
       p->count++;
   }
   
   static void avg_final(sqlite3_context *context) {
       AvgCtx *p = (AvgCtx *)sqlite3_aggregate_context(context, sizeof(AvgCtx));
       if (p->count > 0) {
           sqlite3_result_double(context, p->sum / p->count);
       } else {
           sqlite3_result_null(context);
       }
   }
   
   int sqlite3_myextension_init(sqlite3 *db, char **pzErrMsg, const sqlite3_api_routines *pApi) {
       SQLITE_EXTENSION_INIT2(pApi);
       sqlite3_create_function(db, "my_avg", 1, SQLITE_UTF8, NULL, NULL, avg_step, avg_final);
       return SQLITE_OK;
   }

5. Creating Virtual Tables

   c
   // C code example: Create virtual table module
   typedef struct MyVtab MyVtab;
   struct MyVtab {
       sqlite3_vtab base;
       // Custom data...
   };
   
   static int myCreate(sqlite3 *db, void *pAux, int argc, const char *const *argv, sqlite3_vtab **ppVtab, char **pzErr) {
       MyVtab *pVtab = (MyVtab *)sqlite3_malloc(sizeof(MyVtab));
       // Initialize virtual table...
       *ppVtab = (sqlite3_vtab *)pVtab;
       return SQLITE_OK;
   }
   
   static int myConnect(sqlite3 *db, void *pAux, int argc, const char *const *argv, sqlite3_vtab **ppVtab, char **pzErr) {
       // Connect virtual table...
       return SQLITE_OK;
   }
   
   static int myBestIndex(sqlite3_vtab *pVtab, sqlite3_index_info *pIdxInfo) {
       // Optimize query plan...
       return SQLITE_OK;
   }
   
   static int myFilter(sqlite3_vtab_cursor *pCursor, int idxNum, const char *idxStr, int argc, sqlite3_value **argv) {
       // Filter data...
       return SQLITE_OK;
   }
   
   static int myNext(sqlite3_vtab_cursor *pCursor) {
       // Move to next row...
       return SQLITE_OK;
   }
   
   static int myEof(sqlite3_vtab_cursor *pCursor) {
       // Check if reached end...
       return 0;
   }
   
   static int myColumn(sqlite3_vtab_cursor *pCursor, sqlite3_context *context, int i) {
       // Return column value...
       return SQLITE_OK;
   }
   
   static int myRowid(sqlite3_vtab_cursor *pCursor, sqlite3_int64 *pRowid) {
       // Return row ID...
       return SQLITE_OK;
   }
   
   static sqlite3_module myModule = {
       .iVersion = 0,
       .xCreate = myCreate,
       .xConnect = myConnect,
       .xBestIndex = myBestIndex,
       .xDisconnect = myDisconnect,
       .xDestroy = myDestroy,
       .xOpen = myOpen,
       .xClose = myClose,
       .xFilter = myFilter,
       .xNext = myNext,
       .xEof = myEof,
       .xColumn = myColumn,
       .xRowid = myRowid
   };
   
   int sqlite3_myextension_init(sqlite3 *db, char **pzErrMsg, const sqlite3_api_routines *pApi) {
       SQLITE_EXTENSION_INIT2(pApi);
       sqlite3_create_module(db, "my_module", &myModule, 0);
       return SQLITE_OK;
   }

6. Common Built-in Extensions

   • FTS3/FTS4/FTS5: Full-text search
   • RTREE: Spatial indexing
   • STAT4: Query optimization statistics
   • JSON1: JSON support
   • GEOPOLY: Geographic polygons

7. Extension Application Scenarios

   • Custom business logic functions
   • Integrate third-party libraries
   • Implement special data types
   • Optimize specific query patterns
   • Add encryption support

8. Extension Development Considerations

   • Follow SQLite's memory management rules
   • Properly handle error conditions
   • Ensure thread safety
   • Provide complete documentation
   • Conduct thorough testing

9. Cross-Language Extensions

   • Python: Use sqlite3.create_function()
   • JavaScript: Use Database.createFunction()
   • Java: Use Function.create()
   • Go: Use RegisterFunc()

SQLite's extension mechanism provides powerful extensibility, allowing developers to customize database functionality according to their needs.