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🎨 Create func signatures for the SQLite VTab
plugin/module.go now has the functions signatures to create a virtual table in SQLite. The remaining work is now to implement the function body. **Explanation of the code** As specified in the SQLite documentation (https://www.sqlite.org/vtab.html), a VTab (Virtual Table) is implemented with three structs: - the module struct => Infer the schema and create the connection - the table struct => Find the best querying method and open the cursor struct - the cursor struct => query and update the rows Each of these structs have several methods attached to them. You can refer to the source code to find their exact "raison d'être".
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -1 +1,112 @@ | ||
| package plugin | ||
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| import ( | ||
| "github.com/gammazero/deque" | ||
| "github.com/mattn/go-sqlite3" | ||
| ) | ||
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| // This file links the plugin to the SQLite Virtual Table interface | ||
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| // SQLiteModule is a struct that holds the information about the SQLite module | ||
| // | ||
| // For each table that the plugin provides and for each profile, a new SQLiteModule | ||
| // should be created and registered in the main program | ||
| type SQLiteModule struct { | ||
| PluginPath string | ||
| PluginManifest PluginManifest | ||
| TableIndex int | ||
| client *InternalClient | ||
| } | ||
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| // SQLiteTable that holds the information needed for the BestIndex and Open methods | ||
| type SQLiteTable struct { | ||
| nextCursor int | ||
| tableIndex int | ||
| schema DatabaseSchema | ||
| client *InternalClient | ||
| } | ||
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| // SQLiteCursor holds the information needed for the Column, Filter, EOF and Next methods | ||
| type SQLiteCursor struct { | ||
| tableIndex int | ||
| cursorIndex int | ||
| schema DatabaseSchema | ||
| client *InternalClient | ||
| noMoreRows bool | ||
| rows *deque.Deque[[]interface{}] // A ring buffer to store the rows before sending them to SQLite | ||
| nextCursor *int | ||
| } | ||
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| // EponymousOnlyModule is a method that is used to mark the table as eponymous-only | ||
| // | ||
| // See https://www.sqlite.org/vtab.html#eponymous_virtual_tables for more information | ||
| func (m *SQLiteModule) EponymousOnlyModule() {} | ||
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| // Create is called when the virtual table is created e.g. CREATE VIRTUAL TABLE or SELECT...FROM(epo_table) | ||
| // | ||
| // Its main job is to create a new RPC client and return the needed information | ||
| // for the SQLite virtual table methods | ||
| func (m *SQLiteModule) Create(c *sqlite3.SQLiteConn, args []string) (sqlite3.VTab, error) { | ||
| return nil, nil | ||
| } | ||
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| // Connect is called when the virtual table is connected | ||
| // | ||
| // Because it's an eponymous-only module, the method must be identical to Create | ||
| func (m *SQLiteModule) Connect(c *sqlite3.SQLiteConn, args []string) (sqlite3.VTab, error) { | ||
| return m.Create(c, args) | ||
| } | ||
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| // BestIndex is called when the virtual table is queried | ||
| // to figure out the best way to query the table | ||
| // | ||
| // However, we don't use it that way but only to serialize the constraints | ||
| // for the Filter method | ||
| func (t *SQLiteTable) BestIndex(cst []sqlite3.InfoConstraint, ob []sqlite3.InfoOrderBy) (*sqlite3.IndexResult, error) { | ||
| return nil, nil | ||
| } | ||
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| // Open is called when a new cursor is opened | ||
| // | ||
| // It should return a new cursor | ||
| func (t *SQLiteTable) Open() (sqlite3.VTabCursor, error) { | ||
| return nil, nil | ||
| } | ||
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| // Close is called when the cursor is no longer needed | ||
| func (c *SQLiteCursor) Close() error { return nil } | ||
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| // These methods are not used in this plugin | ||
| func (v *SQLiteTable) Disconnect() error { return nil } | ||
| func (v *SQLiteTable) Destroy() error { return nil } | ||
| func (v *SQLiteModule) DestroyModule() {} | ||
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| // Column is called when a column is queried | ||
| // | ||
| // It should return the value of the column | ||
| func (c *SQLiteCursor) Column(cst int) (interface{}, error) { return nil, nil } | ||
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| // EOF is called after each row is queried to check if there are more rows | ||
| func (c *SQLiteCursor) EOF() bool { return false } | ||
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| // Next is called to move the cursor to the next row | ||
| // | ||
| // If noMoreRows is set to false, and the cursor is at the end of the rows, | ||
| // Next will ask the plugin for more rows | ||
| // | ||
| // If noMoreRows is set to true, Next will set EOF to true | ||
| func (c *SQLiteCursor) Next() error { return nil } | ||
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| // RowID is called to get the row ID of the current row | ||
| func (c *SQLiteCursor) RowID() (int64, error) { return 0, nil } | ||
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| func (c *SQLiteCursor) Filter(idxNum int, idxStr string, vals []interface{}) error { | ||
| // Filter can be called several times with the same cursor | ||
| // Each time, it is supposed to reset the cursor to the beginning | ||
| // Therefore, it should wipe out all the cursor fields | ||
| // | ||
| // Moreover, for the sake of simplicity, we will create a new cursor on the plugin side, | ||
| // which means the cursorIndex must be incremented while not yelding any conflict | ||
| // How to fix this? We must have access to the parent struct (SQLiteTable). | ||
| return nil | ||
| } |
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